a, b = map(int, input().split())

s = a + b
f = 1
ans = 10**18 + 1
while f * f <= s:
    if s % f == 0:
        for c in (f, s // f):
            if (b + c) % a == 0 and (c + a) % b == 0:
                ans = min(ans, f)
    f += 1
if ans == 10**18 + 1:
    ans = -1
print(ans)