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>
namespace Acc {
int a, b, c, d;
void work() {
  scanf("%d%d%d%d", &a, &b, &c, &d);
  printf("%.12f",
         (double)a * d / ((double)a * d + (double)b * c - (double)a * c));
}
}  // namespace Acc
int main() { return Acc::work(), 0; }