2 4
1 17
#include<stdio.h>
__int64 C(int m,int n){
__int64 s=1;
for(int i=n,j=1;j<=m;--i,++j)
s=s*i/j;
return s;
}
__int64 f(int x){
__int64 s=0;
for(int i=2;i<=x;++i){
s+=C(i,x)*(i-1);
}
return s;
}
int main(){
int n;
while(~scanf("%d",&n)){
printf("%I64d\n",f(n));
}
return 0;
}原文：http://blog.csdn.net/qq_18062811/article/details/45076361