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

using namespace std;

int N, M;

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

int minAmount = INT_MAX, currAmount = 0;
long long Summ = 0;

void doSearch() {

}

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

    for (int i = 0; i < 3; i++) {
        currAmount++;
        Summ += i*coinsIn[nCoin];
        usedCoins.push_back(nCoin);

        if (Summ == N) {
            if (currAmount < minAmount) {
                currAmount = minAmount;
            }
        }
        BackTracking(nCoin + 1);
    }
}

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;
}