#include <bits/stdc++.h>
#pragma GCC optimize("Ofast")
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  long long int a, b, c, d;
  cin >> a >> b >> c >> d;
  long double t =
      (a * 1.0 / b) * (1.0 / (1 - (1 - a * 1.0 / b) * (1 - c * 1.0 / d)));
  cout << t << fixed << setprecision(6);
}
