#include <bits/stdc++.h>
using namespace std;
int main() {
  int a, b, c, d;
  cin >> a >> b >> c >> d;
  int T = 1000000;
  double C = (1 - (double)a / b) * (1 - (double)c / d), ans = (double)a / b;
  while (T--) {
    ans += ans * C;
    C *= C;
  }
  printf("%.10lf", (double)ans);
}
