#include <bits/stdc++.h>
using namespace std;
int main() {
  double a, b, c, d;
  scanf("%lf %lf %lf %lf", &a, &b, &c, &d);
  double p, q, r;
  r = a / b;
  p = c / d;
  q = r / (1 - (1 - p) * (1 - r));
  printf("%.10lf\n", q);
}
