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

a,b = map(int,input().split())
cli = make_divisors(a+b)
for i in range(len(cli)):
    c = cli[i]
    if a == c or b == c:
        continue
    if (a+b) % c == 0 and (b+c) % a == 0 and (c+a) % b == 0:
        print(c)
        exit()

print(-1)