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

l = make_divisors(A + B)

for li in l:
    if li == A or li == B:
        continue
    if (li + A) % B == 0 and (li + B) % A == 0:
        print(li)
        break
else:
    print(-1)