#include <bits/stdc++.h>
using namespace std;
using ll = long long;
const int inf = 1e9 + 7;
const ll longinf = 1LL << 60;
const ll mod = 1e9 + 7;
int main() {
  ll a, b;
  cin >> a >> b;
  if (a < b) {
    cout << -1 << endl;
    return 0;
  }
  ll s = a + b;
  ll ng = 3e9, ok = 0;
  while (ng - ok > 1) {
    ll mid = (ok + ng) / 2;
    if (s < 2 * mid * b) {
      ng = mid;
    } else
      ok = mid;
  }
  double ans = 1.0 * s / 2.0 / ok;
  ng = 3e9, ok = 0;
  s = a - b;
  while (ng - ok > 1) {
    ll mid = (ok + ng) / 2;
    if (s < 2 * mid * b) {
      ng = mid;
    } else
      ok = mid;
  }
  ans = min(ans, 1.0 * s / 2.0 / ok);
  cout << fixed << setprecision(12) << ans << endl;
  return 0;
}
