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

using namespace std;
void BackTracking(int nCoin);

int N, M;

vector<int> usedCoins;
vector<int> coinsIn;

int minAmount = M + M + 10, currAmount = 0;
long long Summ = 0;

void doSearch() {
    BackTracking(0);
}

void BackTracking(int nCoin) {
    if (nCoin >= M) return;
    if (Summ > N) return;



    for (int i = 0; i < 3; i++) {

        BackTracking(nCoin + 1);

        if (Summ == N) {
                cout << "!!!!!!";
            if (currAmount < minAmount) {
                minAmount = currAmount;
            }
        }

        if (i != 0) {
            currAmount++;
            Summ += coinsIn[nCoin];
        }

        usedCoins.push_back( coinsIn[nCoin] );

        cout << "i " << i << " nCoin " << nCoin << " Summ " << Summ << " currAmount " << currAmount << endl;

        if (i != 0) {
            currAmount--;
            Summ -= coinsIn[nCoin];
            usedCoins.pop_back();
        }

    }

    /*currAmount -= 2;
    Summ -= 2*coinsIn[nCoin];
    usedCoins.pop_back();
     usedCoins.pop_back();
      usedCoins.pop_back();*/
}

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


    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;
}