There is a polyline going through points (0, 0) – (x, x) – (2x, 0) – (3x, x) – (4x, 0) – ... - (2kx, 0) – (2kx + x, x) – .... 

We know that the polyline passes through the point (a, b). Find minimum positive value x such that it is true or determine that there is no such x.

Input

Only one line containing two positive integers a and b (1 ≤ a, b ≤ 109).

Output

Output the only line containing the answer. Your answer will be considered correct if its relative or absolute error doesn't exceed 10 - 9. If there is no such x then output  - 1 as the answer.

Examples

Input

3 1


Output

1.000000000000


Input

1 3


Output

-1


Input

4 1


Output

1.250000000000

Note

You can see following graphs for sample 1 and sample 3. 

<image> <image>
#include <bits/stdc++.h>
using namespace std;
int main(int argc, char **argv) {
  ios_base::sync_with_stdio(false);
  int a, b;
  cin >> a >> b;
  cout << setprecision(12) << fixed;
  if (a < b) {
    cout << -1 << endl;
    return 0;
  }
  if (a == b) {
    cout << b << endl;
    return 0;
  }
  double ans = -1.0;
  int k = (a - b) / (2 * b);
  if (k > 0) {
    ans = (a - b) / (2.0 * k);
  }
  int kp1 = (a + b) / (2 * b);
  if (kp1 > 0) {
    double res = (a + b) / (2.0 * kp1);
    if (ans < 0) {
      ans = res;
    } else if (ans > res) {
      ans = res;
    }
  }
  cout << ans << endl;
  return 0;
}
