def make_divisors(n):
    divisors = []  # 必要に応じてsetにしても良いかも
    i = 1
    while i ** 2 <= n:
        if n % i == 0:
            divisors.append(i)
            if i ** 2 != n:
                divisors.append(n // i)
        i += 1
    divisors.sort()
    return divisors


A, B = map(int, input().split())
D = make_divisors(A + B)
ans = 10 ** 10
for C in D:
    if C == A or C == B:
        continue
    if (A + C) % B == 0 and (B + C) % A == 0:
        ans = min(ans, C)
if ans == 10 ** 10:
    print(-1)
else:
    print(ans)