#include <bits/stdc++.h>
#pragma comment(linker, "/STACK:256000000")
const int inf = 1e9 + 5;
const long long linf = 1e18;
const long double eps = 1e-10;
const long long mod = 1e9 + 7;
const int maxn = 2e5 + 10;
using namespace std;
int main() {
  ios_base::sync_with_stdio(0);
  double ans;
  double a, b;
  cin >> a >> b;
  if (a < b) {
    cout << -1;
    exit(0);
  }
  if (a == b) {
    cout << a;
    exit(0);
  }
  double s1 = (a - b) / 2;
  double s2 = (a + b) / 2;
  int l = 1;
  int r = inf;
  while (l + 1 != r) {
    int m = (l + r) / 2;
    if (s1 / (double)m + eps < b) {
      r = m;
    } else {
      l = m;
    }
  }
  double ans1 = s1 / l;
  if (ans1 + eps < b) ans1 = inf;
  l = 1;
  r = inf;
  while (l + 1 != r) {
    int m = (l + r) / 2;
    if (s2 / (double)m + eps < b) {
      r = m;
    } else {
      l = m;
    }
  }
  double ans2 = s2 / l;
  cout.precision(10);
  cout << fixed << min(ans1, ans2);
  return 0;
}
