#include <bits/stdc++.h>
using namespace std;
const long double eps = 1e-9;
class solver {
 public:
  long double a, b;
  long long ai, bi;
  void input() {
    cin >> ai >> bi;
    a = ai;
    b = bi;
  }
  void solve() {
    if (b > a) {
      cout << -1 << endl;
      return;
    }
    long long nc = ai / bi;
    if (nc % 2 == 0) --nc;
    long double c1 = nc + 1;
    long double c2 = -a;
    long double c3 = b;
    long double xc = (c1 * c3) / (c3 - c2);
    long double yc = (-c1 * c2) / (c3 - c2);
    long double l1 = sqrt(a * a + b * b);
    long double l2 = sqrt(xc * xc + yc * yc);
    long double ans = l1 / l2;
    cout << fixed;
    cout << setprecision(10) << ans << endl;
  }
};
int main() {
  solver slv;
  slv.input();
  slv.solve();
  return 0;
}
