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>
a, b = [int(i) for i in input().split()]
if (b > a):
    print(-1)
    exit(0)
ans = 0
c = a - b
d = c / b
d = (d // 2) * 2
if d != 0:
    ans = c / d
c = a + b
d = c / b
d = (d // 2) * 2
if ans != 0:
    if c / d < ans:
        ans = c / d
else:
    ans = c / d
print(ans)