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>).
import sys
input = sys.stdin.readline

a, b = map(int, input().split())
f = 10**9
if b > a:
    print(-1)
else:
    c, d = a+b, a-b
    for i in range(2, 10**5, 2):
        x = c/i
        if x >= b:
            f = min(f, x)
        x = d/i
        if x >= b:
            f = min(f, x)
    if f == 10**9:
        print(-1)
    else:
        print(f)