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 = map(float,input().split(" "))
if a==b:
    print(b)
elif a<b:
    print(-1)
else:
    n=(a-b)//b+1
    if n%2==0:n-=1
    if n>1:
        x1=(a-b)/(n-1)
    else: x1=999999999999999999
    n=(a+b)//b-1
    if n%2==0:n-=1
    x2=(a+b)/(n+1)
    print(min(x1,x2))