#include <bits/stdc++.h>
using namespace std;
int main() {
  double a = 0, b = 0, c = 0, d = 0;
  cin >> a >> b >> c >> d;
  double pr = 0.0;
  pr = (a * d / (a * d + b * c - a * c));
  cout << pr << "\n";
}
