#include <bits/stdc++.h>
using namespace std;
int main() {
  double a, b, c, d;
  cin >> a >> b >> c >> d;
  double mx;
  mx = (a / b) * (1 / (1 - (1 - a / b) * (1 - c / d)));
  std::cout << std::setprecision(9) << mx << '\n';
}
