#include <bits/stdc++.h>
using namespace std;
float rec(float x, float y) { return (1 - y) * (1 - x) * x; }
int main() {
  float a, b, c, d;
  cin >> a >> b >> c >> d;
  cout << a / b * (1 / (1 - ((1 - a / b) * (1 - c / d))));
}
