Choose and divide UVA - 10375

待理解，待重做

#include<cstdio>
#include<cstring>
#include<iostream>
#include<algorithm>
using namespace std;
typedef long long ll;

int main()
{   int p,q,r,s;
    while(~scanf("%d%d%d%d",&p,&q,&r,&s)){
        double ans=1.0;
        if(p-q<q) q=p-q;
        if(r-s<s) s=r-s;
        for(int i=1;i<=q||i<=s;i++){
            if(i<=q) ans=ans*(p-q+i)/i;
            if(i<=s) ans=ans/(r-s+i)*i;
        }
        printf("%.5f\n",ans);
     }
     return 0;
}