#include <bits/stdc++.h>
using namespace std;
const int maxx = 1000 * 1000 * 1000;
int bin(double x, int b) {
  int l = 1, r = maxx;
  while (r - l > 1) {
    int c = (l + r) / 2;
    if (x / c > b)
      l = c;
    else
      r = c;
  }
  if (x / r >= b) return r;
  return l;
}
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(NULL);
  cout.tie(NULL);
  int a, b;
  cin >> a >> b;
  if (b > a) {
    cout << -1 << '\n';
    return 0;
  }
  double x1 = (a + b) / 2.0;
  double x2 = (b - a) / 2.0;
  int i = bin(x1, b);
  int j = bin(x2, b);
  cout.precision(10);
  if (x2 <= 0) {
    cout << fixed << x1 / (i) << '\n';
    return 0;
  }
  cout << fixed << min(x1 / (i), x2 / (j)) << '\n';
  return 0;
}
