N, M = map(int, input().split())
A = list(map(int, input().split()))
A.sort()
dic = dict(zip(A, range(N)))

vstd = [False] * N
if A[0] != 0:
    print(-1)
    exit()
AS = set(A)

B = set()
for a in A[1:]:
    if vstd[dic[a]]:
        continue
    for i in range(2, M // a + 1):
        if i * a in AS:
            vstd[dic[i * a]] = True
        else:
            print(-1)
            exit()
    B.add(a)

if len(B) == 0:
    print(-1)
else:
    print(len(B))
    print(*sorted(list(B)))