#include <bits/stdc++.h>
using namespace std;
int main() {
  double a, b, c, d, p, res;
  cin >> a >> b >> c >> d;
  res = a / b;
  p = (1 - a / b) * (1 - c / d);
  p = 1 / (1 - p);
  res *= p;
  cout << fixed << setprecision(12) << res << endl;
  return 0;
}
