#include <bits/stdc++.h>
using namespace std;
int32_t main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  long double a, b, c, d;
  cin >> a >> b >> c >> d;
  long double f = a / b, g = c / d;
  long double x = (1 - f) * (1 - g);
  long double p = f / (1 - x);
  cout << setprecision(20) << p;
  return 0;
}
