#include <bits/stdc++.h>
using namespace std;
int main() {
  float a, b, c, d;
  cin >> a >> b >> c >> d;
  float x = (1 - a / b) * (1 - c / d);
  x = (a / b) * (1 / (1 - x));
  printf("%.12f", x);
  return 0;
}
