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

INF = 10**18
res = INF
n = 0

while n ** 2 <= A + B:
    n += 1
    if (A + B) % n == 0:
        if (A + n) % B == 0 and (B + n) % A == 0 and A != n and B != n:
            res = min(res, n)
        m = (A + B) // n
        if (A + m) % B == 0 and (B + m) % A == 0 and A != m and B != m: 
            res = min(res, (A + B) // n)

print(res if res != INF else -1)