#include <bits/stdc++.h>
using namespace std;

int i;
double a, b, c, d, dist = 1e9, v1, v2;

int main() {
  ios_base::sync_with_stdio(0);

  cin >> a >> b >> c >> d;

  v1 = dist / a;
  v2 = dist / c;
  v1 *= b; v2 *= d;

  cout << setprecision(7) << fixed << dist / (v1 + v2) << '\n';

  return 0;
}