#include <bits/stdc++.h>
using namespace std;
#define FOR(i,m,n) for(int i=(m);i<(n);++i)
#define REP(i,n) FOR(i,0,n)
using ll = long long;
constexpr int INF = 0x3f3f3f3f;
constexpr long long LINF = 0x3f3f3f3f3f3f3f3fLL;
constexpr double EPS = 1e-8;
constexpr int MOD = 998244353;
// constexpr int MOD = 1000000007;
constexpr int DY4[]{1, 0, -1, 0}, DX4[]{0, -1, 0, 1};
constexpr int DY8[]{1, 1, 0, -1, -1, -1, 0, 1};
constexpr int DX8[]{0, -1, -1, -1, 0, 1, 1, 1};
template <typename T, typename U>
inline bool chmax(T& a, U b) { return a < b ? (a = b, true) : false; }
template <typename T, typename U>
inline bool chmin(T& a, U b) { return a > b ? (a = b, true) : false; }
struct IOSetup {
  IOSetup() {
    std::cin.tie(nullptr);
    std::ios_base::sync_with_stdio(false);
    std::cout << fixed << setprecision(20);
  }
} iosetup;

int main() {
  int n, m; cin >> n >> m;
  set<int> a;
  while (n--) {
    int a_i; cin >> a_i;
    a.emplace(a_i);
  }
  a.erase(a.begin());
  const set<int> cp = a;
  const int last = *a.rbegin();
  vector<int> b;
  while (!a.empty()) {
    const int beat = *a.begin();
    b.emplace_back(beat);
    for (int i = beat; i <= last; i += beat) {
      if (a.erase(i) == 0 && !cp.contains(i)) {
        cout << "-1\n";
        return 0;
      }
    }
  }
  const int l = b.size();
  cout << l << '\n';
  REP(i, l) cout << b[i] << " \n"[i + 1 == l];
  return 0;
}