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