POJ 1306

其实求的这个数的式子化简一下，就是C(N,M).....

#include <iostream>
#include <algorithm>
#include <cstdio>
#define LL __int64
using namespace std;

LL N,M;

LL myc(LL n,LL r){
	LL sum=1;
	for(LL i=1;i<=r;i++)
	sum=sum*(n-r+i)/i;
	return sum;
}

int main(){
	while(scanf("%d%d",&N,&M),N||M){
		LL ans=myc(N,M);
		printf("%I64d things taken %I64d at a time is %I64d exactly.\n",N,M,ans);
	}
	return 0;
}