def make_divisors(n):
    divisors = []
    for i in range(1, int(n**0.5)+1):
        if n % i == 0:
            divisors.append(i)
            if i != n // i:
                divisors.append(n//i)

    # divisors.sort()
    return divisors


def main():
    a, b = map(int, input().split())
    l = make_divisors(a + b)
    ans = float("inf")
    for c in l:
        if a == c or b == c:
            continue
        if (b + c) % a == 0 and (a + c) % b == 0:
            ans = min(ans, c)
    if ans == float("inf"):
        print(-1)
    else:
        print(ans)


if __name__ == '__main__':
    main()