#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  double a, b, c, d;
  while (cin >> a >> b >> c >> d) {
    cout << setprecision(12) << fixed
         << (a / b) / (1.0 - (1.0 - (a / b)) * (1.0 - (c / d))) << endl;
  }
  return 0;
}
