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:
l=lambda:map(int,raw_input().split())
a,b=l()
if a<b:
    print -1
    exit()
if a==b:
    print "%.12f" %b
    exit() 
y=(a+b)/(2*b)
ans1=0.5*(a+b)/y 
y=(a-b)/(2*b)
if y<>0:
    ans2=0.5*(a-b)/y
    print "%.12f" %min(ans1,ans2)   
else: 
    print "%.12f" %ans1