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