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>).
a, b = map(int, input().split())
if a == b:
    print(a)
elif a < b:
    print(-1)
else:
    x = 1
    y = 1
    while (a + b) / (2 * x) > b:
        x += 1
    while (a - b) / (2 * y) > b:
        y += 1
    x -= 1
    y -= 1
    print(min((a + b) / (2 * x), (a + b) / (2 * y)))
