#include <bits/stdc++.h>
using namespace std;
int main() {
  double a, b, c, d;
  cin >> a >> b >> c >> d;
  double a1 = a / b;
  double a2 = (1 - c / d) * (1 - a / b);
  double ans = a1 / (1 - a2);
  cout << ans << endl;
  return 0;
}
