#include <iostream>
#include <algorithm>
#include <functional>
#include <string>
#include <climits>
#include <vector>
#include <numeric>
#include <complex>
#include <map>
#include <bitset>
using namespace std;

//#define __int64 long long
#define long __int64
#define REP(i,a,b) for(int i=a;i<b;i++)
#define rep(i,n) REP(i,0,n)
const int Vecy[4] = {0,-1,0,1};
const int Vecx[4] = {1,0,-1,0};

int x[100000] = {0};
int total,n;
int a[100000] = {0};

bool func(int op, int left, int &ans){
	if(left == 0){
		ans = accumulate(x, x+n, 0);
		return true;
	}
	if(op >= n){
		return false;
	}
	
	x[op] = left / a[op];
	
	while(a[op] >= 0){
		if(func(op+1, left - a[op] * x[op], ans)){
			return true;
		}
		
		if(x[op] == 0){
			return false;
		}
		x[op] --;
	}
	x[op] = 0;
	return false;
}

int main(){
	int ans = 0;
	
	cin >> total >> n;
	rep(i,n){
		cin >> a[i];
	}
	
	sort(a, a + n, greater<int>());  	//降順
	
	if(func(0, total, ans)){
		cout << ans << endl;
	}else{
		cout << -1 << endl;
	}
	
    return 0;
}