#include <bits/stdc++.h>
using namespace std;
int main() {
  float a, b, c, d, rat1, rat2, prob, res, lob, hor;
  cin >> a >> b >> c >> d;
  rat1 = (1 - (a / b)) * (1 - (c / d));
  prob = a / b;
  prob = prob / (1 - rat1);
  printf("%.6f\n", prob);
}
