#include <bits/stdc++.h>
using namespace std;
int main() {
  long long int a, b, c, d;
  cin >> a >> b >> c >> d;
  double ans = a * d;
  ans /= (b * d - (b - a) * (d - c));
  cout.precision(12);
  cout << ans;
  return 0;
}
