10491 - Cows and Cars

描述：要么全选择牛，要么选择一辆车和p-1头牛，那么剩下n+m-p道门可以选择，求选择p道门以后要选择到车的概率#include <cstdio>int main(){    //freopen("in.txt","r",stdin);    int m,n,p;    while(scanf("%d%d%d",&m,&n,&p)!=EOF)    {        printf("%.5lf\n",(n*m*1.0+(n-1)*n*1.0)/(m+n)/(m+n-p-1));    }    return 0;}