How Many Points of Intersection?

uva10790:http://uva.onlinejudge.org/index.php?option=com_onlinejudge&Itemid=8&page=show_problem&problem=1731

题意：两条水平线，分别有n,m个点，点之间两两连线，求有多少个交点。

题解：手动模拟一下，然后用不完全归纳法，就可以得到公式ans=(n-1)*n/2*(m-1)*m/2;

#include<iostream>
#include<cstdio>
#include<algorithm>
#include<cstring>
using namespace std;
long long m, n;
int main(){
    int tt=1;
     while(~scanf("%lld%lld",&n,&m)){
        if(n==0&&m==0)break;
       printf("Case %d: %lld\n",tt++,(n-1)*n/2*(m-1)*(m)/2);
     }
}