N,M = map(int, input().split())
A = list(map(int, input().split()))
A.sort()
from collections import defaultdict
d = defaultdict(lambda:0)
s_A = set(A)
ans = []
for i in A[1:]:

    if d[i] > 0:
        continue

    for j in range(i,M+1,i):
        if j not in s_A:
            print(-1)
            exit()
        else:
            d[j] += 1
    ans.append(i)
    #print(d)
print(len(ans))
print(*ans)