Problem: 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>

Solution (in CPP):

Solution:
#include <bits/stdc++.h>
using namespace std;
int main() {
  int a, b, chushu, s, n, x;
  double h1, h2;
  cin >> a >> b;
  chushu = b;
  if (a < b) {
    cout << "-1" << endl;
    return 0;
  }
  if (a == b) {
    cout << a << endl;
    return 0;
  }
  if (b == 0) {
    cout << "0" << endl;
    return 0;
  }
  x = b;
  s = a - b;
  n = s / x;
  if (n % 2 != 0) n--;
  h1 = 1.0 * s / n;
  s = a + b;
  n = s / x;
  if (n % 2 != 0) n--;
  h2 = 1.0 * s / n;
  if (h1 > h2) h1 = h2;
  printf("%.12lf\n", h1);
  return 0;
}