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() {
  ios_base::sync_with_stdio(0);
  double a, b, c, d;
  double x1, x2, y, x;
  double ans = 0;
  double e = 1E-8;
  cin >> a >> b >> c >> d;
  x1 = a / b;
  y = (d - c) / d;
  x2 = (b - a) / b;
  x = 1;
  while (x > e) {
    ans += (x * x1);
    x *= x2;
    x *= y;
  }
  cout << fixed;
  cout.precision(9);
  cout << ans << endl;
  return 0;
}