from collections import defaultdict
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]

a, b = map(int, input().split())
m = make_divisors(a+b)
for c in m:
    if (a+b)%c == 0 and (b+c)%a == 0 and (c+a)%b == 0:
        print(c)
        exit()
print(-1)