def div(n):
    if n <= 0:
        return []
    S = set()
    i = 1
    while i * i <= n:
        if n % i == 0:
            S.add(i)
            S.add(n // i)
        i += 1
    return list(S)

A, B = map(int, input().split())
inf = 10 ** 18
ans = inf
for d in div(A + B):
    if (A + d) % B == 0 and (B + d) % A == 0:
        ans = min(ans, d)
print(ans) if ans != inf else print(-1)