#include <bits/stdc++.h>
using namespace std;
int main() {
  double a, b, c, d, r;
  cin >> a >> b >> c >> d;
  r = ((1 - (a / b)) * ((1 - (c / d))));
  printf("%.7lf\n", (a / b) / (1 - r));
  return 0;
}
