#include <bits/stdc++.h>
using namespace std;
long long MOD = 1e9 + 7;
void solve() {
  long long a, b;
  cin >> a >> b;
  if (a < b) {
    cout << -1 << "\n";
    return;
  }
  if (a == b) {
    cout << a << "\n";
    return;
  }
  double d = (a - b) / 2.0;
  long long k = d / b;
  double ans1 = d / k;
  d = (b + a) / 2.0;
  k = d / b;
  double ans2 = d / k;
  std::cout << std::fixed << std::setprecision(12) << min(ans1, ans2) << "\n";
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  int t = 1;
  while (t--) {
    solve();
  }
}
