光棍节的快乐 NYOJ 451

#include<stdio.h>//光棍节的快乐（451） 
long long f(int x)
{
    long long res=1,i;
    for(i=1;i<=x;i++){
        res=res*i;
    }
    return res;
}
long long g(int x)
{
    if(x==2)return 1;
    else if(x==3)return 2;
    else return (x-1)*(g(x-1)+g(x-2));
}
int main()
{
    long long  m,n,t,res,i;
    while(scanf("%lld%lld",&m,&n)!=EOF){
        t=m-n;
        res=f(m)/(f(t)*f(n));
        printf("%lld\n",res*g(n));
    }
    return 0;
}