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>).
ent = input().split()
x = int(ent[0])
y = int(ent[1])

qn = ((x + y)/(2 * y)).__trunc__()
if qn == 0:
    print(-1)
else:
    n = (x + y)/(2 * qn)
    print(n)
