hdu 4465 Candy 2012 成都现场赛

/**
对于大数的很好的应用，，缩小放大，，保持精度
**/
#include <iostream>
#include <cmath>
#include <algorithm>
#include <cstdio>
using namespace std;

int main()
{
    double n,p;
    int cnt =1;
    while(cin>>n>>p){
        double p1 = log(p+0.0);
        double p2 = log(1-p+0.0);
        double s1 = (n+1)*p1;
        double s2 = (n+1)*p2;
        double ans =0,c=0;
        for(int i=0;i<n;i++){
            ans += (exp(c+s1)+exp(c+s2))*(n-i);
            c += log(n+1+i+0.0)-log(i+1+0.0);
            s1 += p2;
            s2 += p1;
        }
        printf("Case %d: %.6lf\n",cnt++,ans);
    }
    return 0;
}