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())

def solve(a, b):
    if a < b:
        return -1
    k0 = (a - b)//(2 * b)
    x0 = (a - b)/(2 * k0)
    k1 = (a + b)//(2 * b)
    x1 = (a + b)/(2 * k1)
    return min(x0, x1)

print(solve(a, b))
