N,M=map(int,input().split())
A=sorted(list(map(int,input().split()))[1:])
use={a:False for a in A}
ans=[]
for a in A:
	if use[a]:
		continue
	ans.append(a)
	for i in range(a,M+1,a):
		if i not in use:
			print(-1)
			exit()
		use[i]=True
print(len(ans))
print(*ans)