#include <bits/stdc++.h>
using namespace std;
int main() {
  int a, b;
  scanf("%d%d", &a, &b);
  if (b > a)
    puts("-1");
  else if (b == a)
    printf("%.12lf\n", (double)a);
  else {
    double t = (a - b) / 2.0;
    t = a - t;
    long long l = 1;
    long long r = 1e18;
    while (r > l) {
      long long mid = (l + r + 1ll) >> 1ll;
      if (t / mid >= b)
        l = mid;
      else
        r = mid - 1;
    }
    t /= (1. * l);
    printf("%.12lf\n", (double)t);
  }
  return 0;
}
