N, M = map(int, input().split())
A = list(map(int, input().split()))
A.sort()
stA = set(A)

stA.discard(0)

if A[1] == 1:
    if M+1 == N:
        print(1)
        print(1)
        exit()
    else:
        print(-1)
        exit()


ansset = set()
ans = []
for p in A:
    if p not in ansset and p != 0:
        ans.append(p)
        for i in range(p, M+1, p):
            ansset.add(i)

if ansset == stA:
    print(len(ans))
    print(*ans)
else:
    print(-1)