def divisors(n: int) -> list[int]:
    """n の約数を求める"""
    s = set()
    p = 1
    while p * p <= n:
        if n % p == 0:
            s.add(p)
            s.add(n // p)
        p += 1
    return sorted(s)


A, B = map(int, input().split())

for c in divisors(A+B):
    if (A+c) % B == 0 and (B+c) % A == 0 and A != c and B != c:
        print(c)
        break
else:
    print(-1)