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

//* ATCODER
#include<atcoder/all>
using namespace atcoder;
typedef modint998244353 mint;
//*/

/* BOOST MULTIPRECISION
#include<boost/multiprecision/cpp_int.hpp>
using namespace boost::multiprecision;
//*/

typedef long long ll;

#define rep(i, s, n) for (int i = (int)(s); i < (int)(n); i++)
#define rrep(i, s, n) for (int i = (int)(n)-1; i >= (int)(s); i--)

template <typename T> bool chmin(T &a, const T &b) {
	if (a <= b) return false;
	a = b;
	return true;
}

template <typename T> bool chmax(T &a, const T &b) {
	if (a >= b) return false;
	a = b;
	return true;
}

template <typename T> T max(vector<T> &a){
	assert(!a.empty());
	T ret = a[0];
	for (int i=0; i<(int)a.size(); i++) chmax(ret, a[i]);
	return ret;
}

template <typename T> T min(vector<T> &a){
	assert(!a.empty());
	T ret = a[0];
	for (int i=0; i<(int)a.size(); i++) chmin(ret, a[i]);
	return ret;
}

template <typename T> T sum(vector<T> &a){
	T ret = 0;
	for (int i=0; i<(int)a.size(); i++) ret += a[i];
	return ret;
}

int main(){
	ll n, m;
	cin >> n >> m;
	vector<ll> a(n);
	rep(i,0,n){
		cin >> a[i];
	}
	sort(a.begin(), a.end());
	priority_queue<pair<ll,ll>> pq;
	rep(i,1,n){
		if (pq.empty()){
			pq.push(pair(- a[i] * 2, a[i]));
			continue;
		}
		if (- pq.top().first < a[i]){
			cout << -1 << '\n';
			return 0;
		}
		if (!(- pq.top().first == a[i])){
			pq.push(pair(-a[i] * 2, a[i]));
		}else{
			while(- pq.top().first == a[i]){
				auto [x, y] = pq.top();
				pq.pop();
				pq.push(pair(x-y, y));
			}
		}
	}

	vector<ll> ans;
	while(!pq.empty()){
		auto [x, y] = pq.top();
		pq.pop();
		ans.push_back(y);
	}
	sort(ans.begin(), ans.end());
	
	int k = ans.size();
	cout << k << endl;
	rep(i,0,k){
		cout << ans[i] << ' ';
	}
	cout << '\n';
}