#include <bits/stdc++.h>
double a, b, c, d, ans, ans1, e, f;
long g, h;
int main() {
  scanf("%lf %lf", &a, &b);
  if (a < b)
    printf("-1");
  else if (a == b)
    printf("%.12lf", a);
  else {
    c = a + b;
    d = a - b;
    e = c / b;
    f = d / b;
    g = e;
    h = f;
    if (d < b) d = 1010101010;
    if (g % 2 != 0) g--;
    if (h % 2 != 0) h--;
    ans = c * 1.0 / (g * 1.0);
    ans1 = d * 1.0 / (h * 1.0);
    if (ans < ans1)
      printf("%.12lf", ans);
    else
      printf("%.12lf", ans1);
  }
}
