#include <bits/stdc++.h>
using namespace std;
int main() {
  double a, b, c, d, b1, q;
  cin >> a >> b >> c >> d;
  b1 = a / b;
  q = (1 - a / b) * (1 - c / d);
  double ans = b1 / (1 - q);
  printf("%.9lf", ans);
}
