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 b <= (a + b) // 2:
    k = 1
    for i in range(30):
        nk = k + (1 << i)
        if b <= (a + b) // (2 * nk):
            k = nk
    x = (a + b) / (2 * k)
    print('{:.10f}'.format(x))
else:
    print(-1)
