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
from math import ceil
[a,b] = list(map(float,input().split()))
if a < b:
    print(-1)
elif a == b:
    print(a)
else:
    a1 = (a + b)/2
    k1 = ceil(a1/b)
    x1 = a1/k1
    a2 = (a - b)/2
    k2 = ceil(a2/b)
    x2 = a2/k2
    print(min(x1,x2))
