import sys


def input(): return sys.stdin.readline().rstrip('\n')


def main():
    a, b = map(int, input().split())
    for c in [abs(a - b), a + b]:
        if a != b != c != a and (a + b) % c == 0 and (b + c) % a == 0 and (c + a) % b == 0:
            return c
    return -1


if __name__ == '__main__':
    ret = main()
    if ret is not None:
        print(ret)