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

# a + b = 0 mod c
# c は a+b の約数である
def make_divisors(n):
    lower_divisors , upper_divisors = [], []
    i = 1
    while i*i <= n:
        if n % i == 0:
            lower_divisors.append(i)
            if i != n // i:
                upper_divisors.append(n//i)
        i += 1
    return lower_divisors + upper_divisors[::-1]

divs = make_divisors(A + B)

for c in divs:
    if (B + c)%A == 0 and (c + A)%B == 0:
        exit(print(c))

print(-1)