from math import sqrt, floor


def calc_positive_divisors(num: int) -> list[int]:
    small_divisors = []
    large_divisors = []
    for n in range(1, floor(sqrt(num))+1):
        if num % n == 0:
            small_divisors.append(n)
            large_divisors.append(num//n)
    if small_divisors[-1] == large_divisors[-1]:
        large_divisors.pop()
    divisors = small_divisors + list(reversed(large_divisors))
    return divisors


def main():
    A, B = map(int, input().split())
    sum_divisors = calc_positive_divisors(A+B)
    A_rest = A % B
    B_rest = B % A
    for divisor in sum_divisors:
        if divisor in (A, B):
            continue
        if not (A_rest + divisor % B) % B and not (B_rest + divisor % A) % A:
            print(divisor)
            return
    print(-1)


if __name__ == "__main__":
    main()