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 a, b, c, d;
int main() {
  cin >> a >> b >> c >> d;
  double ans = 0.0;
  double r = 1;
  double p = (double)a / b;
  double q = (double)c / d;
  p = 1 - p;
  q = 1 - q;
  for (int i = 0; i < 1000007; i++) {
    ans += (r / b) * a;
    r *= (p * q);
  }
  printf("%.12f\n", ans);
  return 0;
}