#include <bits/stdc++.h>
using namespace std;

#define rep(i, n) for (int i = 0; i < (int)n; i++)

int main() {
  int n, m;
  cin >> n >> m;
  map<int, int> mp;
  rep(i, n) {
    int a;
    cin >> a;
    mp[a]++;
  }
  vector<int> b;
  bool ans = true;
  for (map<int, int>::iterator it = mp.begin(); it != mp.end(); it++) {
    int key = it->first;
    if (key == 0)
      continue;
    if (mp[key] == 1) {
      b.push_back(key);
      for (int i = 1; i * key <= m; i++) {
	if (mp[i * key] == 0)
	  ans = false;
	else
	  mp[i * key]++;
      }
    }
  }
  if (ans) {
    cout << (int)b.size() << endl;
    for (int i : b)
      cout << i << " ";
    cout << endl;
  } else
    cout << -1 << endl;
  return 0;
}