#include <bits/stdc++.h>
using namespace std;
using Int = long long;
template <typename T1, typename T2>
inline void chmin(T1 &a, T2 b) {
  if (a > b) a = b;
}
template <typename T1, typename T2>
inline void chmax(T1 &a, T2 b) {
  if (a < b) a = b;
}
struct FastIO {
  FastIO() {
    cin.tie(0);
    ios::sync_with_stdio(0);
  }
} fastio_beet;
template <typename T>
void drop(const T &x) {
  cout << x << endl;
  exit(0);
}
struct Precision {
  Precision() { cout << fixed << setprecision(12); }
} precision_beet;
signed main() {
  Int a, b;
  cin >> a >> b;
  if (a < b) drop(-1);
  using D = double;
  if (a == b) drop(D(a));
  D s = a + b, t = a - b;
  D ans = s / 2.0;
  Int p = (a + b) / (2 * b);
  chmin(ans, s / (2 * p));
  Int q = (a - b) / (2 * b);
  chmin(ans, t / (2 * q));
  cout << ans << endl;
  return 0;
}
