#include <bits/stdc++.h>
using namespace std;
int main() {
  double a, b, c, d;
  cin >> a >> b >> c >> d;
  double prob = 0;
  double at = 1.0;
  double p1acertar = (a / b);
  double p2errar = (1 - c / d);
  for (int i = 0; i < 100000; i++) {
    prob += at * p1acertar;
    at *= (1 - p1acertar);
    at *= p2errar;
  }
  printf("%.8lf\n", prob);
  return 0;
}
