#include <bits/stdc++.h>
using namespace std;
int main() {
  int a, b, c, d;
  cin >> a >> b >> c >> d;
  long double p = (1.0 * a / b);
  long double q = (1 - (1.0 * a / b)) * (1 - (1.0 * c / d));
  cout.precision(12);
  cout << fixed << (1.0 * p / (1.0 - q)) << endl;
  return 0;
}
