diff --git a/build.sbt b/build.sbt
index 3b2ee7f..b100bb5 100644
--- a/build.sbt
+++ b/build.sbt
@@ -16,5 +16,6 @@ lazy val root = project
       "-unchecked",
       "-Wunused:all"
     ),
-    libraryDependencies += "org.scalameta" %% "munit" % "0.7.29" % Test
+    libraryDependencies += "org.scalameta" %% "munit" % "0.7.29" % Test,
+    Test / parallelExecution := false
   )
diff --git a/src/main/scala/io/github/acl4s/FenwickTree.scala b/src/main/scala/io/github/acl4s/FenwickTree.scala
new file mode 100644
index 0000000..b9171ef
--- /dev/null
+++ b/src/main/scala/io/github/acl4s/FenwickTree.scala
@@ -0,0 +1,127 @@
+package io.github.acl4s
+
+import scala.reflect.ClassTag
+
+import io.github.acl4s.internal.rightOpenInterval
+
+/**
+ * Reference: https://en.wikipedia.org/wiki/Fenwick_tree
+ *
+ * @param n
+ * @param m
+ * @tparam T
+ */
+case class FenwickTree[T: ClassTag](n: Int)(using m: AddSub[T]) {
+  private val data: Array[T] = Array.fill(n)(m.e())
+
+  def add(index: Int, x: T): Unit = {
+    assert(0 <= index && index < n)
+    var p = index + 1
+    while (p <= n) {
+      data(p - 1) = m.combine(data(p - 1), x)
+      p += p & -p
+    }
+  }
+
+  private def sum(i: Int): T = {
+    var s = m.e()
+    var r = i
+    while (r > 0) {
+      s = m.combine(s, data(r - 1))
+      r -= r & -r
+    }
+    s
+  }
+
+  def sum(range: Range): T = {
+    val (l, r) = rightOpenInterval(range)
+    sum(l, r)
+  }
+
+  def sum(l: Int, r: Int): T = {
+    assert(0 <= l && l <= r && r <= n)
+    m.subtract(sum(r), sum(l))
+  }
+}
+
+object FenwickTree {
+  def apply[T: AddSub: ClassTag](array: Array[T]): FenwickTree[T] = {
+    val ft = FenwickTree[T](array.length)
+    array.indices.foreach(i => {
+      ft.add(i, array(i))
+    })
+    ft
+  }
+}
+
+trait AddSub[T] extends Add[T] {
+  def subtract(a: T, b: T): T
+}
+
+object AddSub {
+  given (using m: Add[Char]): AddSub[Char] with {
+    override def e(): Char = m.e()
+    override def combine(a: Char, b: Char): Char = m.combine(a, b)
+    override def subtract(a: Char, b: Char): Char = (a - b).asInstanceOf[Char]
+  }
+
+  given (using m: Add[Byte]): AddSub[Byte] with {
+    override def e(): Byte = m.e()
+    override def combine(a: Byte, b: Byte): Byte = m.combine(a, b)
+    override def subtract(a: Byte, b: Byte): Byte = (a - b).asInstanceOf[Byte]
+  }
+
+  given (using m: Add[Short]): AddSub[Short] with {
+    override def e(): Short = m.e()
+    override def combine(a: Short, b: Short): Short = m.combine(a, b)
+    override def subtract(a: Short, b: Short): Short = (a - b).asInstanceOf[Short]
+  }
+
+  given (using m: Add[Int]): AddSub[Int] with {
+    override def e(): Int = m.e()
+    override def combine(a: Int, b: Int): Int = m.combine(a, b)
+    override def subtract(a: Int, b: Int): Int = a - b
+  }
+
+  given (using m: Add[Long]): AddSub[Long] with {
+    override def e(): Long = m.e()
+    override def combine(a: Long, b: Long): Long = m.combine(a, b)
+    override def subtract(a: Long, b: Long): Long = a - b
+  }
+
+  given (using m: Add[Float]): AddSub[Float] with {
+    override def e(): Float = m.e()
+    override def combine(a: Float, b: Float): Float = m.combine(a, b)
+    override def subtract(a: Float, b: Float): Float = a - b
+  }
+
+  given (using m: Add[Double]): AddSub[Double] with {
+    override def e(): Double = m.e()
+    override def combine(a: Double, b: Double): Double = m.combine(a, b)
+    override def subtract(a: Double, b: Double): Double = a - b
+  }
+
+  given (using m: Add[DynamicModInt]): AddSub[DynamicModInt] with {
+    override def e(): DynamicModInt = m.e()
+    override def combine(a: DynamicModInt, b: DynamicModInt): DynamicModInt = m.combine(a, b)
+    override def subtract(a: DynamicModInt, b: DynamicModInt): DynamicModInt = a - b
+  }
+
+  given (using m: Add[ModInt998244353]): AddSub[ModInt998244353] with {
+    override def e(): ModInt998244353 = m.e()
+    override def combine(a: ModInt998244353, b: ModInt998244353): ModInt998244353 = m.combine(a, b)
+    override def subtract(a: ModInt998244353, b: ModInt998244353): ModInt998244353 = a - b
+  }
+
+  given (using m: Add[ModInt1000000007]): AddSub[ModInt1000000007] with {
+    override def e(): ModInt1000000007 = m.e()
+    override def combine(a: ModInt1000000007, b: ModInt1000000007): ModInt1000000007 = m.combine(a, b)
+    override def subtract(a: ModInt1000000007, b: ModInt1000000007): ModInt1000000007 = a - b
+  }
+
+  given [T <: Int](using m: Add[StaticModInt[T]]): AddSub[StaticModInt[T]] with {
+    override def e(): StaticModInt[T] = m.e()
+    override def combine(a: StaticModInt[T], b: StaticModInt[T]): StaticModInt[T] = m.combine(a, b)
+    override def subtract(a: StaticModInt[T], b: StaticModInt[T]): StaticModInt[T] = a - b
+  }
+}
diff --git a/src/main/scala/io/github/acl4s/ModInt.scala b/src/main/scala/io/github/acl4s/ModInt.scala
index 0b1f52c..6756af1 100644
--- a/src/main/scala/io/github/acl4s/ModInt.scala
+++ b/src/main/scala/io/github/acl4s/ModInt.scala
@@ -189,6 +189,7 @@ type ModInt998244353 = StaticModInt[Mod998244353.value.type]
 object ModInt1000000007 {
   given Modulus[1_000_000_007] = Mod1000000007
 
+  def apply(): ModInt1000000007 = StaticModInt()
   def apply(value: Int): ModInt1000000007 = StaticModInt(value)
   def apply(value: Long): ModInt1000000007 = StaticModInt(value)
 }
@@ -196,6 +197,7 @@ object ModInt1000000007 {
 object ModInt998244353 {
   given Modulus[998_244_353] = Mod998244353
 
+  def apply(): ModInt998244353 = StaticModInt()
   def apply(value: Int): ModInt998244353 = StaticModInt(value)
   def apply(value: Long): ModInt998244353 = StaticModInt(value)
 }
diff --git a/src/main/scala/io/github/acl4s/Monoid.scala b/src/main/scala/io/github/acl4s/Monoid.scala
index 3db742e..91c5cf0 100644
--- a/src/main/scala/io/github/acl4s/Monoid.scala
+++ b/src/main/scala/io/github/acl4s/Monoid.scala
@@ -8,3 +8,67 @@ trait Monoid[T] {
 object Monoid {
   def apply[T](using m: Monoid[T]): Monoid[T] = m
 }
+
+trait Add[T] extends Monoid[T]
+object Add {
+  given Add[Char] with {
+    override def e(): Char = 0
+    override def combine(a: Char, b: Char): Char = (a + b).asInstanceOf[Char]
+  }
+
+  given Add[Byte] with {
+    override def e(): Byte = 0
+    override def combine(a: Byte, b: Byte): Byte = (a + b).asInstanceOf[Byte]
+  }
+
+  given Add[Short] with {
+    override def e(): Short = 0
+    override def combine(a: Short, b: Short): Short = (a + b).asInstanceOf[Short]
+  }
+
+  given Add[Int] with {
+    override def e(): Int = 0
+    override def combine(a: Int, b: Int): Int = a + b
+  }
+
+  given Add[Long] with {
+    override def e(): Long = 0L
+    override def combine(a: Long, b: Long): Long = a + b
+  }
+
+  given Add[Float] with {
+    override def e(): Float = 0f
+    override def combine(a: Float, b: Float): Float = a + b
+  }
+
+  given Add[Double] with {
+    override def e(): Double = 0d
+    override def combine(a: Double, b: Double): Double = a + b
+  }
+
+  given Add[DynamicModInt] with {
+    private val zero = DynamicModInt()
+    override def e(): DynamicModInt = zero
+    override def combine(a: DynamicModInt, b: DynamicModInt): DynamicModInt = a + b
+  }
+
+  given Add[ModInt998244353] with {
+    private val zero = ModInt998244353()
+    override def e(): ModInt998244353 = zero
+    override def combine(a: ModInt998244353, b: ModInt998244353): ModInt998244353 = a + b
+  }
+
+  given Add[ModInt1000000007] with {
+    private val zero = ModInt1000000007()
+    override def e(): ModInt1000000007 = zero
+    override def combine(a: ModInt1000000007, b: ModInt1000000007): ModInt1000000007 = a + b
+  }
+
+  given [T <: Int](using Modulus[T]): Add[StaticModInt[T]] with {
+    private val zero = StaticModInt[T]()
+    override def e(): StaticModInt[T] = zero
+    override def combine(a: StaticModInt[T], b: StaticModInt[T]): StaticModInt[T] = a + b
+  }
+
+  def apply[T](using m: Add[T]): Add[T] = m
+}
diff --git a/src/test/scala/io/github/acl4s/FenwickTreeSuite.scala b/src/test/scala/io/github/acl4s/FenwickTreeSuite.scala
new file mode 100644
index 0000000..4a63b41
--- /dev/null
+++ b/src/test/scala/io/github/acl4s/FenwickTreeSuite.scala
@@ -0,0 +1,170 @@
+package io.github.acl4s
+
+class FenwickTreeSuite extends munit.FunSuite {
+
+  /**
+   * @see https://atcoder.jp/contests/practice2/tasks/practice2_b
+   */
+  test("AtCoder Library Practice Contest B - Fenwick Tree") {
+    val fw = FenwickTree(Array(1L, 2L, 3L, 4L, 5L))
+
+    assertEquals(fw.sum(0, 5), 15L)
+    assertEquals(fw.sum(2, 4), 7L)
+
+    fw.add(3, 10)
+
+    assertEquals(fw.sum(0, 5), 25L)
+    assertEquals(fw.sum(0, 3), 6L)
+  }
+
+  test("zero") {
+    {
+      val fw = FenwickTree[Long](0)
+      assertEquals(fw.sum(0, 0), 0L)
+    }
+
+    {
+      type ModInt = DynamicModInt
+      val ModInt = DynamicModInt
+
+      val fw = FenwickTree[ModInt](0)
+      assertEquals(fw.sum(0, 0), ModInt(0))
+    }
+
+    {
+      type ModInt = ModInt998244353
+      val ModInt = ModInt998244353
+
+      val fw = FenwickTree[ModInt](0)
+      assertEquals(fw.sum(0, 0), ModInt(0))
+    }
+
+    {
+      type ModInt = ModInt1000000007
+      val ModInt = ModInt1000000007
+
+      val fw = FenwickTree[ModInt](0)
+      assertEquals(fw.sum(0, 0), ModInt(0))
+    }
+
+    {
+      given Modulus[1_000_000_009] = Modulus[1_000_000_009]()
+      type ModInt = StaticModInt[1_000_000_009]
+      val ModInt = StaticModInt
+
+      val fw = FenwickTree[ModInt](0)
+      assertEquals(fw.sum(0, 0), ModInt(0))
+    }
+  }
+
+  test("naive") {
+    (0 to 50).foreach(n => {
+      val fw = FenwickTree[Long](n)
+      (0 until n).foreach(i => {
+        fw.add(i, i.toLong * i)
+      })
+
+      for {
+        l <- 0 to n
+        r <- l to n
+      } {
+        val sum = (l until r).map(i => i.toLong * i).sum
+        assertEquals(fw.sum(l, r), sum)
+      }
+    })
+  }
+
+  test("bound int") {
+    val fw = FenwickTree[Int](10)
+
+    fw.add(3, Int.MaxValue)
+    fw.add(5, Int.MinValue)
+
+    assertEquals(fw.sum(0, 10), -1)
+    assertEquals(fw.sum(3, 6), -1)
+
+    assertEquals(fw.sum(3, 4), Int.MaxValue)
+    assertEquals(fw.sum(4, 10), Int.MinValue)
+  }
+
+  test("bound long") {
+    val fw = FenwickTree[Long](10)
+
+    fw.add(3, Long.MaxValue)
+    fw.add(5, Long.MinValue)
+
+    assertEquals(fw.sum(0, 10), -1L)
+    assertEquals(fw.sum(3, 6), -1L)
+
+    assertEquals(fw.sum(3, 4), Long.MaxValue)
+    assertEquals(fw.sum(4, 10), Long.MinValue)
+  }
+
+  test("overflow") {
+    val fw = FenwickTree[Int](20)
+    val a = new Array[Long](20)
+    (0 until 10).foreach(i => {
+      fw.add(i, Int.MaxValue)
+      a(i) += Int.MaxValue
+    })
+    (10 until 20).foreach(i => {
+      fw.add(i, Int.MinValue)
+      a(i) += Int.MinValue
+    })
+
+    fw.add(5, 11_111)
+    a(5) += 11_111
+
+    for {
+      l <- 0 to 20
+      r <- l to 20
+    } {
+      val sum = (l until r).map(i => a(i)).sum
+      val dif = sum - fw.sum(l, r)
+      assertEquals(dif % (1L << 32), 0L)
+    }
+  }
+
+  test("StaticModInt") {
+    given Modulus[11] = Modulus[11]()
+    type ModInt = StaticModInt[11]
+    val ModInt = StaticModInt
+
+    (0 to 50).foreach(n => {
+      val fw = FenwickTree[ModInt](n)
+      (0 until n).foreach(i => {
+        fw.add(i, ModInt(i.toLong * i))
+      })
+
+      for {
+        l <- 0 to n
+        r <- l to n
+      } {
+        val sum = (l until r).map(i => i.toLong * i).sum
+        assertEquals(fw.sum(l, r), ModInt(sum))
+      }
+    })
+  }
+
+  test("DynamicModInt") {
+    type ModInt = DynamicModInt
+    val ModInt = DynamicModInt
+    ModInt.setMod(11)
+
+    (0 to 50).foreach(n => {
+      val fw = FenwickTree[ModInt](n)
+      (0 until n).foreach(i => {
+        fw.add(i, ModInt(i.toLong * i))
+      })
+
+      for {
+        l <- 0 to n
+        r <- l to n
+      } {
+        val sum = (l until r).map(i => i.toLong * i).sum
+        assertEquals(fw.sum(l, r), ModInt(sum))
+      }
+    })
+  }
+
+}