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;
  while (~scanf("%lf%lf", &x, &y)) {
    if (x < y) {
      printf("-1\n");
    } else if (x == y) {
      printf("%.9lf\n", x);
    } else {
      double k1 = floor((x - y) / 2 / y);
      double k2 = floor((x + y) / 2 / y);
      printf("%.9lf\n", min((x - y) / k1 / 2, (x + y) / k2 / 2));
    }
  }
}