#include <bits/stdc++.h>
using namespace std;
int a, b, c, d;
int main() {
  cin >> a >> b >> c >> d;
  double ans = 0.0;
  double r = 1;
  double p = (double)a / b;
  double q = (double)c / d;
  p = 1 - p;
  q = 1 - q;
  for (int i = 0; i < 1000007; i++) {
    ans += (r / b) * a;
    r *= (p * q);
  }
  printf("%.12f\n", ans);
  return 0;
}
