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() {
  double x, y;
  cin >> x >> y;
  if (x < y) {
    printf("-1\n");
  } else if (x == y) {
    printf("%.9lf\n", x);
  } else {
    double k1 = floor((x - y) * 0.5 / y);
    double k2 = floor((x + y) * 0.5 / y);
    printf("%.12f\n", min((x - y) * 0.5 / k1, (x + y) * 0.5 / k2));
  }
}