N, M = map(int, input().split())
A = sorted(list(map(int, input().split())))
S = set()
ans = []
for a in A[1:]:
    if a not in S:
        ans.append(a)
        for i in range(a, M + 1, a):
            S.add(i)
            if len(S) > N:
                print(-1)
                exit()
                
S.add(0)
S = sorted(list(S))
if A == S:
    print(len(ans))
    print(*ans)
else:
    print(-1)