#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

int main(){
    int C, N;
    cin >> C >> N;
    vector<int> a(N);
    for(int i = 0; i < N; i++){
        cin >> a[i];
    }
    for(int i = 1; i < a.size(); i++){
        if(a[i] == a[i-1]){
            a.erase(a.begin()+i);
            i--;
        }
    }
    N = a.size();
    vector<vector<int> > dp(N+1, vector<int>(C+1, 1000*1000*1000));
    dp[0][0] = 0;
    for(int i = 0; i < N; i++){
        for(int j = 0; j <= C; j++){
            dp[i+1][j] = dp[i][j];
        }
        for(int j = 0; j < a[i]; j++){
            if(dp[i][j] == 1000*1000*1000) continue;
            int cost = dp[i][j];
            for(int k = j; k <= C; k += a[i]){
                dp[i+1][k] = min(dp[i+1][k], cost);
                cost = dp[i+1][k] + 1;
                //cerr << i << " : " << k << " : " << dp[i+1][k] << endl;
            }
        }
    }
    if(dp[N][C] == 1000*1000*1000){
        cout << -1 << endl;
    }else{
        cout << dp[N][C] << endl;
    }
    return 0;
}