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>
import decimal

a, b = map(int, input().split())

a = decimal.Decimal(a)
b = decimal.Decimal(b)

if a<b:
    print('-1')
else:
    k = a//b
    ans = round((a+b)/k,12)
    #print('1: '+str(ans))
    #print('2: '+str(((a+b)/(k+1))))
    if k%2==1:
        ans = round((a+b)/(k+1),12)
    print(ans)