#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(0);
  int a, b, c, d;
  cin >> a >> b >> c >> d;
  double pS = (double)a / b;
  double pZ = (double)c / d;
  double r = (1 - pS) * (1 - pZ);
  double res = pS * (1 / (1 - r));
  cout << fixed << setprecision(10) << res;
}
