CodeForces 312B Archer

Archer

Time Limit: 2000ms

Memory Limit: 262144KB

This problem will be judged on CodeForces. Original ID: 312B
64-bit integer IO format: %I64d      Java class name: (Any)

 

SmallR is an archer. SmallR is taking a match of archer with Zanoes. They try to shoot in the target in turns, and SmallR shoots first. The probability of shooting the target each time is for SmallR while  for Zanoes. The one who shoots in the target first should be the winner.

Output the probability that SmallR will win the match.

 

Input

A single line contains four integers .

 

Output

Print a single real number, the probability that SmallR will win the match.

The answer will be considered correct if the absolute or relative error doesn't exceed 10 - 6.

 

Sample Input

Input

1 2 1 2

Output

0.666666666667

Source

Codeforces Round #185 (Div. 2)

 

解题：等比数列求和的极限

 

 #include <bits/stdc++.h>
 using namespace std;
 int a,b,c,d;
 int main(){
     while(~scanf("%d%d%d%d",&a,&b,&c,&d))
         printf("%.8f\n",(double)a*d/(a*d+b*c-c*a));
     return ;
 }