#include <bits/stdc++.h>
using namespace std;
set<string> st;
map<string, long long> mp;
int main() {
  double a, b, c, d, N, D;
  cin >> a >> b >> c >> d;
  N = a * d;
  D = b * c + a * d - a * c;
  cout << fixed << setprecision(12) << N / D;
  return 0;
}
