diff --git a/src/main/java/shoe/example/firstproject/App.java b/src/main/java/shoe/example/firstproject/App.java
deleted file mode 100644
index 431a954..0000000
--- a/src/main/java/shoe/example/firstproject/App.java
+++ /dev/null
@@ -1,7 +0,0 @@
-package shoe.example.firstproject;
-
-public class App {
-    public static void main(String[] args) {
-        System.out.println("Hello World!");
-    }
-}
diff --git a/src/main/java/shoe/example/firstproject/ShuntingYardAlgorithm.java b/src/main/java/shoe/example/firstproject/ShuntingYardAlgorithm.java
index 3ec1ef1..0f3dd3b 100644
--- a/src/main/java/shoe/example/firstproject/ShuntingYardAlgorithm.java
+++ b/src/main/java/shoe/example/firstproject/ShuntingYardAlgorithm.java
@@ -1,7 +1,11 @@
 package shoe.example.firstproject;
 
+
 public class ShuntingYardAlgorithm {
     public String translate(String infixExpression) {
-        return infixExpression != null ? infixExpression : "";
+        String result = infixExpression != null ? infixExpression : "";
+        if(!result.matches("\\d*"))
+            throw new IllegalArgumentException("Badly formed expression");
+        return result;
     }
 }
diff --git a/src/test/java/shoe/example/firstproject/AppTest.java b/src/test/java/shoe/example/firstproject/AppTest.java
deleted file mode 100644
index 3a12d70..0000000
--- a/src/test/java/shoe/example/firstproject/AppTest.java
+++ /dev/null
@@ -1,35 +0,0 @@
-package shoe.example.firstproject;
-
-import org.junit.After;
-import org.junit.Before;
-import org.junit.Test;
-
-import java.io.ByteArrayOutputStream;
-import java.io.PrintStream;
-
-import static org.hamcrest.CoreMatchers.is;
-import static org.junit.Assert.assertThat;
-
-public class AppTest {
-    private ByteArrayOutputStream byteArrayOutputStream;
-    private PrintStream originalSystemOut;
-
-    @Before
-    public void redirectOutput() {
-        byteArrayOutputStream = new ByteArrayOutputStream();
-        PrintStream printStream = new PrintStream(byteArrayOutputStream);
-        originalSystemOut = System.out;
-        System.setOut(printStream);
-    }
-
-    @After
-    public void restoreOutput() {
-        System.setOut(originalSystemOut);
-    }
-
-    @Test
-    public void testApp() {
-        App.main(null);
-        assertThat(byteArrayOutputStream.toString(), is("Hello World!\n"));
-    }
-}
diff --git a/src/test/java/shoe/example/firstproject/ShuntingYardAlgorithmTest.java b/src/test/java/shoe/example/firstproject/ShuntingYardAlgorithmTest.java
index 336e323..c237e9f 100644
--- a/src/test/java/shoe/example/firstproject/ShuntingYardAlgorithmTest.java
+++ b/src/test/java/shoe/example/firstproject/ShuntingYardAlgorithmTest.java
@@ -31,4 +31,9 @@ public void shouldReturnNumberUnchanged() {
         String result = shuntingYardAlgorithm.translate("45");
         assertThat(result, is("45"));
     }
+
+    @Test(expected=RuntimeException.class)
+    public void shouldRejectOperatorOnly() {
+        shuntingYardAlgorithm.translate("+");
+    }
 }