There is a polyline going through points (0, 0) – (*x*, *x*) – (2*x*, 0) – (3*x*, *x*) – (4*x*, 0) – ... - (2*kx*, 0) – (2*kx* + *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*.
Only one line containing two positive integers *a* and *b* (1 ≤ *a*, *b* ≤ 10<sup class="upper-index">9</sup>).
#578A
[a,b] = list(map(float,input().split()))
if a < b:
    print(-1)
elif a == b:
    print(a)
else:
    a1 = a + b
    a2 = a - b
    while(a1/2 >= b):
        a1 /= 2.
    while(a2/2 >= b):
        a2 /= 2.
    print(min(a1,a2))
