#include <bits/stdc++.h>
using namespace std;
int main() {
  int a, b, c, d;
  while (~scanf("%d%d%d%d", &a, &b, &c, &d)) {
    double p = 1.0 * a / b, q = 1.0 * c / d;
    printf("%.8f\n", p / (1 - (1 - p) * (1 - q)));
  }
  return 0;
}
