N, M = map(int, input().split())
A = sorted(list(map(int, input().split())))

S = set(A[1:])
used = set()
ans = []
for i in range(1, N):
    if A[i] not in S:
        continue
    ans.append(A[i])
    for j in range(A[i], M+1, A[i]):
        if j not in S and j not in used:
            exit(print(-1))
        if j not in used:
            S.remove(j)
            used.add(j)

print(len(ans))
print(*ans)