#include <bits/stdc++.h>
using namespace std;
int main() {
  double p, q;
  int a, b, c, d;
  scanf("%d%d%d%d", &a, &b, &c, &d);
  p = a * 1.0 / b;
  q = c * 1.0 / d;
  printf("%.12lf\n", p / (p + q - p * q));
  return 0;
}
