def divisor_list(N): #約数のリスト
    if N == 1: return [1]
    res = []
    for i in range(1,N):
        if i*i >= N: break
        if N%i == 0:
            res.append(i)
            res.append(N//i)
    if i*i == N: res.append(i)
    return sorted(res)

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