#include <bits/stdc++.h>
using namespace std;
int main() {
  double a, b, x, y;
  double prob;
  cin >> a >> b >> x >> y;
  double ax = (b - a) / b, xy = (y - x) / y;
  a = a / b;
  prob = a / (1 - ax * xy);
  cout << prob;
  return 0;
}
