#include <bits/stdc++.h>
int main() {
  int a, c, b, d;
  double x, y, p, ans;
  scanf("%d%d%d%d", &a, &b, &c, &d);
  x = (double)a / b;
  y = (double)c / d;
  p = (1 - x) * (1 - y);
  ans = x / (1 - p);
  printf("%lf", ans);
  return 0;
}
