#include<bits/stdc++.h>
namespace {
#pragma GCC diagnostic ignored "-Wunused-function"
#include<atcoder/all>
#pragma GCC diagnostic warning "-Wunused-function"
using namespace std;
using namespace atcoder;
#define rep(i,n) for(int i = 0; i < (int)(n); i++)
#define rrep(i,n) for(int i = (int)(n) - 1; i >= 0; i--)
#define all(x) begin(x), end(x)
#define rall(x) rbegin(x), rend(x)
template<class T> bool chmax(T& a, const T& b) { if (a < b) { a = b; return true; } else return false; }
template<class T> bool chmin(T& a, const T& b) { if (b < a) { a = b; return true; } else return false; }
using ll = long long;
using P = pair<int,int>;
using VI = vector<int>;
using VVI = vector<VI>;
using VL = vector<ll>;
using VVL = vector<VL>;

} int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  int n, m;
  cin >> n >> m;
  VI a(n);
  rep(i, n) cin >> a[i];
  sort(all(a));
  a.erase(a.begin());
  set<int> sa(all(a)), todo(all(a));
  VI ans;
  while (todo.size()) {
    int x = *todo.begin();
    ans.emplace_back(x);
    for (int xk = x; xk <= m; xk += x) {
      if (!sa.contains(xk)) {
        cout << -1 << '\n';
        return 0;
      }
      todo.erase(xk);
    }
  }
  int sz = ans.size();
  cout << sz << '\n';
  rep(i, sz) cout << ans[i] << " \n"[i + 1 == sz];
}