#include <bits/stdc++.h>
using namespace std;
const double EPS = 1e-9;
const int INF = 1 << 30;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  double a, b, c, d, p, q;
  cin >> a >> b >> c >> d;
  double smallr = a / b, zannoes = c / d;
  p = smallr, q = (1 - smallr) * (1 - zannoes);
  cout << fixed << setprecision(12) << p / (1 - q);
  return 0;
}
