#include <stdio.h>
#include <bits/stdc++.h>
#include <atcoder/all>
using namespace atcoder;
using mint = modint998244353;
using namespace std;
#define rep(i,n) for (int i = 0; i < (n); ++i)
#define Inf32 1000000001
#define Inf64 1000000000000000001

int main(){

	int N,M;
	cin>>N>>M;
	set<long long> S;
	
	rep(i,N){
		int t;
		cin>>t;
		S.insert(t);
	}
	auto T = S;
	vector<long long> ans;
	while(S.size()>0){
		auto it = S.begin();
		long long v = *it;
		S.erase(it);
		if(v==0)continue;
		ans.push_back(v);
		for(long long j=v*2;j<=M;j+=v){
			if(!T.count(j)){
				cout<<-1<<endl;
				return 0;
			}
			S.erase(j);
		}
	}
	cout<<ans.size()<<endl;
	rep(i,ans.size()){
		if(i!=0)cout<<' ';
		cout<<ans[i];
	}
	cout<<endl;
	return 0;
}