poj1741，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;
}