#include <bits/stdc++.h>
using namespace std;
int main() {
  double a, b, c, d;
  cin >> a >> b >> c >> d;
  double D = (1 - a / b) * (1 - c / d);
  double A = a / b;
  double ans = A / (1 - D);
  cout << setprecision(8) << ans;
}
