Problem: 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 <image> for SmallR while <image> 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 <image>.

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.

Examples

Input

1 2 1 2


Output

0.666666666667

Solution (in CPP):

Solution:
#include <bits/stdc++.h>
using namespace std;
int main() {
  float a, b, c, d;
  cin >> a >> b >> c >> d;
  double ps = a / b, pz = c / d, pw = 0;
  double nps = 1 - ps, npz = 1 - pz;
  for (int i = 0;; i++) {
    double k = pow(nps, i) * pow(npz, i) * ps;
    pw += k;
    if (k < 0.0000000000001) break;
  }
  printf("%.12f", pw);
}