#if __INCLUDE_LEVEL__ == 0

#include __BASE_FILE__

namespace {

void Solve() {
  Int n, m;
  Scan(n, m);
  std::map<Int, Int> mp;
  for (Int i : Rep(n)) {
    Int a;
    Scan(a);
    if (i) {
      mp[a] = 0;
    }
  }
  std::vector<Int> b;
  for (auto [x, y] : mp) {
    if (y) {
      continue;
    }
    b.push_back(x);
    for (Int i = x; i <= m; i += x) {
      if (!mp.contains(i)) {
        Print(-1);
        return;
      }
      ++mp[i];
    }
  }
  Print(Sz(b));
  Print(b);
}

}  // namespace

int main() {
  std::ios::sync_with_stdio(false);
  std::cin.tie(nullptr);
  std::cout << std::setprecision(std::numeric_limits<Float>::max_digits10);

  Solve();
}

#else  // __INCLUDE_LEVEL__

#include <bits/stdc++.h>

using Int = int64_t;
using Float = double;

namespace ranges = std::ranges;
namespace views = std::views;

inline constexpr auto Rev = views::reverse;
inline constexpr auto Map = views::transform;
inline constexpr auto Filter = views::filter;

constexpr auto Rep(Int l, Int r) { return views::iota(std::min(l, r), r); }
constexpr auto Rep(Int n) { return Rep(0, n); }
constexpr auto Rep1(Int l, Int r) { return Rep(l, r + 1); }
constexpr auto Rep1(Int n) { return Rep(1, n + 1); }

constexpr Int Sz(auto&& r) { return static_cast<Int>(ranges::size(r)); }

template <class T, class U = T> bool Chmin(T& x, U&& y) { return y < x && (x = std::forward<U>(y), true); }
template <class T, class U = T> bool Chmax(T& x, U&& y) { return x < y && (x = std::forward<U>(y), true); }

inline constexpr Int INF = [] { std::array<char, sizeof(Int)> a; a.fill(0x3f); return std::bit_cast<Int>(a); }();

template <class T> concept Range = ranges::range<T> && !std::convertible_to<T, std::string_view>;
template <class T> concept TupleLike = std::__is_tuple_like<T>::value && !Range<T>;

namespace std {

istream& operator>>(istream& is, Range auto&& r) {
  for (auto&& e : r) {
    is >> e;
  }
  return is;
}

istream& operator>>(istream& is, TupleLike auto&& t) {
  return apply([&](auto&... xs) -> istream& { return (is >> ... >> xs); }, t);
}

ostream& operator<<(ostream& os, Range auto&& r) {
  for (string_view sep = ""; auto&& e : r) {
    os << exchange(sep, " ") << e;
  }
  return os;
}

ostream& operator<<(ostream& os, TupleLike auto&& t) {
  const auto f = [&](auto&... xs) -> ostream& {
    [[maybe_unused]] string_view sep = "";
    ((os << exchange(sep, " ") << xs), ...);
    return os;
  };
  return apply(f, t);
}

}  // namespace std

#define DEF_INC_OR_DEC(op) \
  auto& operator op(Range auto&& r) { \
    for (auto&& e : r) { \
      op e; \
    } \
    return r; \
  } \
  auto& operator op(TupleLike auto&& t) { \
    std::apply([](auto&... xs) { (op xs, ...); }, t); \
    return t; \
  }

DEF_INC_OR_DEC(++)
DEF_INC_OR_DEC(--)

#undef DEF_INC_OR_DEC

void Scan(auto&&... xs) { std::cin >> std::tie(xs...); }
void Print(auto&&... xs) { std::cout << std::tie(xs...) << '\n'; }

template <class F>
class Fix {
 public:
  explicit Fix(F f) : f_(std::move(f)) {}
  decltype(auto) operator()(auto&&... xs) const { return f_(std::ref(*this), std::forward<decltype(xs)>(xs)...); }

 private:
  F f_;
};

#endif  // __INCLUDE_LEVEL__