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

A, B = map(int, input().split())
D = divisors(A + B)
for C in D:
    if (A + C) % B == 0 and (B + C) % A == 0 and A != C and B != C:
        print(C)
        exit()
print(-1)