def divisors(n):
    ans = []
    i = 1
    while i * i <= n:
        if n % i == 0:
            ans.append(i)
            if i != n//i:
                ans.append(n//i)
        i += 1

    ans.sort()
    return ans

a, b = map(int, input().split())

d = divisors(a+b)
glist = list(set(d))

for i in glist:
    if (a + i) % b == 0 and (b + i) % a == 0 and a != i and b != i:
        print(i)
        break
else:
    print(-1)