#include <bits/stdc++.h>
using namespace std;
int main() {
  int a, b, c, d;
  scanf("%d%d%d%d", &a, &b, &c, &d);
  double p = 1.0 * a / b;
  double q = 1.0 * c / d;
  double r = p / (1 - (1 - p) * (1 - q));
  printf("%.9lf", r);
}
