# oj t -c "python3 main.py"
import sys,math
from collections import defaultdict,deque
from itertools import combinations,permutations,accumulate,product
from bisect import bisect_left,bisect_right
from heapq import heappop,heappush,heapify
#from sortedcontainers import SortedList,SortedSet
def input():return sys.stdin.readline().rstrip()
def ii(): return int(input())
def ms(): return map(int, input().split())
def li(): return list(map(int,input().split()))
inf = pow(10,18)
mod = 998244353
#//////////////////////////////////
N,M = ms()
A = li()
A.sort()
AS = set(A)
ans = list()
ok = set()
ng = set()
def judge(a):
  for j in range(2,a+1):
    if a%j!=0: continue
    if j in ng: continue
    tmp = j
    while tmp<=A[-1]:
      if tmp not in AS:
        ng.add(j)
        break
      tmp += j
    if j in ng: continue
    else: 
      tmp = j
      while tmp<=A[-1]:
        ok.add(tmp)
        tmp += j
      return j
  return -1
  
for i in range(1,N):
  if A[i] in ok: continue
  res = judge(A[i])
  if res==-1:
    print(-1)
    exit()
  ans.append(res)
ans.sort()
print(len(ans))
print(*ans)