/
1204E.go
46 lines (42 loc) · 845 Bytes
/
1204E.go
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package main
import (
. "fmt"
"io"
)
// github.com/EndlessCheng/codeforces-go
func CF1204E(in io.Reader, out io.Writer) {
const mod = 998244853
pow := func(x, n int64) (res int64) {
res = 1
for ; n > 0; n >>= 1 {
if n&1 > 0 {
res = res * x % mod
}
x = x * x % mod
}
return res
}
var n, m int
Fscan(in, &n, &m)
F := make([]int64, n+m+1)
F[0] = 1
for i := 1; i <= n+m; i++ {
F[i] = F[i-1] * int64(i) % mod
}
invF := make([]int64, n+m+1)
invF[n+m] = pow(F[n+m], mod-2)
for i := n + m; i > 0; i-- {
invF[i-1] = invF[i] * int64(i) % mod
}
C := func(n, k int) int64 { return F[n] * invF[k] % mod * invF[n-k] % mod }
ans := int64(0)
for i := 1; i <= n; i++ {
if i > n-m {
ans += C(n+m, n-i)
} else {
ans += C(n+m, n)
}
}
Fprint(out, ans%mod)
}
//func main() { CF1204E(os.Stdin, os.Stdout) }