#include <bits/stdc++.h>
using namespace std;
int main() {
  double a, b, c, d;
  cin >> a >> b >> c >> d;
  double p = a / b;
  double q = c / d;
  double s_k = (1 - p) * (1 - q);
  double prev = p, cur = p + p * s_k;
  while (abs(prev - cur) >= 0.00000000001) {
    prev = cur;
    s_k = s_k * (1 - p) * (1 - q);
    cur = cur + s_k * p;
  }
  printf("%8.7f\n", cur);
  return 0;
}
