Submission #72411578

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Source Code Expand

#include <bits/stdc++.h>
using namespace std;
const int MX=20100100,md=998244353;
int n,m;
long long F[MX],coef2[MX];
long long pw(long long a, int b) {
  if (b==0) return 1LL;
  if (b&1) return (pw(a,b-1)*a)%md;
  long long x=pw(a,b/2);
  return (x*x)%md;
}
int main() {
  scanf("%d%d",&n,&m);
  for (int i=F[0]=1; i<=2*n; i++) F[i]=(F[i-1]*i)%md;
  if (m>n) { puts("0"); return 0; }
  int i=m;
  if ((i&1)!=(n&1)) ++i;
  int rst=(n-i)/2;
  long long coef1=F[i+m-1]*pw(F[2*m-1],md-2)%md*pw(F[rst],md-2)%md;
  coef2[n]=pw(F[n-m],md-2)%md;
  for (int i=n-2; i>=m; i-=2) {
    rst=(n-i)/2;
    coef2[i]=coef2[i+2]*(i+2-m)%md*(i+1-m)%md*(rst+m-1LL)%md;
  }
  long long ans=(coef1*coef2[i])%md;
  coef1=coef1*rst%md;
  for (i+=2; i<=n; i+=2) {
    rst=(n-i)/2;
    coef1=coef1*(i+m-2LL)%md*(i+m-1LL)%md;
    ans=(ans+coef1*coef2[i])%md;
    coef1=coef1*rst%md;
  }
  printf("%lld\n",(ans*pw(2,m))%md);
  return 0;
}

Submission Info

Judge Result