【推导】计蒜客17119 2017 ACM-ICPC 亚洲区（西安赛区）网络赛 F. Trig Function

#include<cstdio>
#include<algorithm>
#include<queue>
#include<set>
using namespace std;
long long n,k,i,ans;
long long mo=998244353;
long long ni(long long a,long long b)
{
    long long c;
    if (b==1) return a;
    c=ni(a*a%mo,b/2);
    if (b%2==1) c=c*a%mo;
    return c;
}

int main(){
    //freopen("ac.in","r",stdin);
    while (scanf("%lld%lld",&n,&k)!=EOF)
    {
        if (n<k || (n+k)%2==1)
        {
            printf("0\n");
            continue;
        }
        ans=n%mo;//
        for (i=n-k+2;i<=n+k-2;i+=2) ans=ans*i%mo;
        for (i=2;i<=k;i++) ans=ans*ni(i,mo-2)%mo;
        if (k==0) ans=ans*ni(n%mo,mo-2)%mo;
        if ((n-k)/2%2==1) ans=mo-ans;
        printf("%lld\n",ans);
    }
}