#include <iostream>
#include <cmath>
#include <algorithm>
#include <math.h>
#include <vector>
#include <fstream>

using namespace std;

vector<int> usedCoins;
int minAmount = 0, currAmount = 0;

void doSearch() {

}

int main()
{
    int N, M;
    long long sum;
    cin >> N >> M;

    vector<int> coinsIn;


    for (int i = 0; i < M; ++i) {
        int t;
        cin >> t;
        coinsIn.push_back(t);
        sum += t*2;
    }

    if (sum < N) {
        cout << "-1";
    } else if (sum == N) {
        cout << coinsIn.size()*2 << endl;
        for (int i = 0; i < coinsIn.size(); i++) {
            cout << coinsIn[i] << " " << coinsIn[i] << " ";
        }
    } else {
        doSearch();
        cout << minAmount << endl;
        for (int i = 0; i < minAmount; i++) {
            cout << usedCoins[i] << " ";
        }
    }


    return 0;
}