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 PYTHON):

Solution:
from math import *
from Queue import *
from sys import *




a, b = map(int, raw_input().split())
if a < b:
    print(-1)
    exit(0)
odd = float(a+b)
eve = float(a-b)
k = (a+b)/(2*b)
res = odd/(2*k)
if a - b >= 2*b:
    k = (a-b)/(2*b)
    res = min(res, eve/(2*k))
print(res)