a, b = map(int, input().split())
def make_divisors(n):
    divisors = []
    for i in range(1, int(n**0.5)+1):
        if n % i == 0:
            divisors.append(i)
            if i != n // i:
                divisors.append(n//i)

    divisors.sort()
    return divisors

d = make_divisors(a+b)
for c in d:
    if (a+c)%b == 0 and (b+c)%a == 0:
        if a != c and b != c:
            print(c)
            exit()
else:
    print(-1)