#include <bits/stdc++.h>
using namespace std;
int main() {
  long double a, b, c, d;
  cin >> a >> b >> c >> d;
  long double p1 = a / b, p2 = c / d;
  cout << p1 / (1 - (1 - p1) * (1 - p2));
  return 0;
}
