#include "bits/stdc++.h"

using namespace std;

int mod = 1000007;
int main() {
    int n,k;
    cin >> n >> k;
    long long arr[n],from[n];
    for(int i = 0; i < n; i++)
    {
        arr[i] = 0;
        from[i] = 0;
    }

    set<int> st;
    int temp;
    for(int i = 0; i < k; i++) { cin >> temp; st.insert(temp); }

    //156

    for(int i = 0; i < n; i++)
    {
        long long ans = -1;// = arr[i-1]+1;
        int fr = -1;//1;
        if(st.count(i+1)) { ans = 0; fr = i+1;}
        for(auto x : st)
        {
            if(i >= x) {
                if(ans == -1 || arr[i-x] < ans)
                {
                    ans = arr[i-x];
                    fr = x;
                }
            }
        }
        arr[i] = ans+1;
        from[i] = fr;
    }

    stack<int> stack;
    cout << arr[n-1] << '\n';
    if(arr[n-1] == -1) return 0;
    int cur_pos = n-1;
    while(cur_pos >= 0)
    {
        stack.push(from[cur_pos]);
        cur_pos -= from[cur_pos];
    }
    while(stack.size() > 0)
    {
        cout << stack.top() << ' ';
        stack.pop();
    }
}