#include <bits/stdc++.h>
using namespace std;
int main() {
  double a, b, c, d;
  cin >> a >> b >> c >> d;
  double p = a / b, q = c / d, r = (1 - p) * (1 - q);
  cout << setprecision(15) << p / (1 - r);
  return 0;
}
