NYOJ 光棍节的快乐 南工451

题目链接：here~~

求组合数X错排数

#include <iostream>
#include <cstdio>
using namespace std;
double myc(int n, int m)
{
double sum;
sum = 1;
for (int i=1; i<=m; i++)
sum=sum*(n-i+1)/i;
// cout<<sum<<endl;
return sum;
}
int main()
{
int n, m;
long long a[21]={0, 0, 1};
for (int i=3; i<=20; i++)
a[i]=(i-1)*(a[i-1]+a[i-2]);
while (cin>>n>>m)
{
n=myc(n, m);
printf("%lld\n", n*a[m]);
}
return 0;
}