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())
CD = make_divisors(A+B)
for C in CD:
    if not A==C and not B==C and (A+C)%B==0 and (B+C)%A==0:
        print(C)
        exit()

print(-1)