#include <bits/stdc++.h>
using namespace std;
int a, b;
double Xcu, Xms;
double len() {
  if (Xcu / 2 < b) return -1;
  int l = 0, r = 1000000007;
  while (r - l > 1) {
    int middle = ((l + r) >> 1);
    double x = Xcu / (middle * 2);
    if (x >= b)
      l = middle;
    else
      r = middle;
  }
  return Xcu / (l * 2);
}
double xuong() {
  if (Xms / 2 < b) return -1;
  int l = 0, r = 1000000007;
  while (r - l > 1) {
    int middle = ((l + r) >> 1);
    double x = Xms / (middle * 2);
    if (x >= b)
      l = middle;
    else
      r = middle;
  }
  return Xms / (l * 2);
}
int main() {
  ios_base::sync_with_stdio(0);
  cin >> a >> b;
  Xcu = a - b, Xms = a + b;
  double x = len();
  double y = xuong();
  if (x == -1 && y == -1) {
    cout << -1;
    return 0;
  }
  if (x == -1) {
    cout << fixed << setprecision(9) << y;
    return 0;
  }
  if (y == -1) {
    cout << fixed << setprecision(9) << x;
    return 0;
  }
  cout << fixed << setprecision(9) << min(x, y);
  return 0;
}
