#include <bits/stdc++.h>
using namespace std;
int main() {
  double a, b, c, d;
  cin >> a >> b >> c >> d;
  double p, q, ans;
  q = ((b - a) / b) * ((d - c) / d);
  p = (a / b);
  ans = p / (1 - q);
  cout << ans;
}
