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 i;
double a, b, c, d, pa, push_back, sc = 1.0, sol;
int main() {
  cin.sync_with_stdio(0);
  cout.sync_with_stdio(0);
  cin >> a >> b >> c >> d;
  pa = a / b;
  push_back = c / d;
  for (i = 1; i <= 1000000; i++) {
    sol += sc * pa;
    sc *= (1 - pa) * (1 - push_back);
  }
  cout << fixed << setprecision(10) << sol;
  return 0;
}