#include<iostream>
#include<cstdio>
#include<cstring>
#include<string>
#include<cctype>
#include<cstdlib>
#include<algorithm>
#include<bitset>
#include<vector>
#include<list>
#include<deque>
#include<queue>
#include<map>
#include<set>
#include<stack>
#include<cmath>
#include<sstream>
#include<fstream>
#include<iomanip>
#include<ctime>
#include<complex>
#include<functional>
#include<climits>
#include<cassert>
#include<iterator>
#include<immintrin.h>
#include<unordered_map>
using namespace std;
int c;
int n;
#define MAX 100002
vector<int> v;
vector<pair<int,int> > dp[100002];
int su[MAX];
int main(){
	scanf("%d", &c);
	scanf("%d", &n);
	for (int i = 0; i < n; i++){
		int a;
		scanf("%d", &a);
		v.push_back(a);
	}
	sort(v.begin(), v.end());
	v.erase(unique(v.begin(), v.end()),v.end());
	dp[0].push_back(make_pair(0, 0));
	for (int i = 0; i < v.size(); i++){
		for (int j = 0; j <= c; j++){
			int ind = j%v[i];
			int ind2 = upper_bound(dp[ind].begin(), dp[ind].end(), make_pair(j, INT_MAX)) - dp[ind].begin();
			ind2--;
			if (ind2 < 0){
				su[j] = INT_MAX;
				continue;
			}
			int val = dp[ind][ind2].first;
			int sa = abs(val - j);
			sa /= v[i];
			sa += dp[ind][ind2].second;
			su[j] = sa;
		}
		for (int i = 0; i < MAX; i++){
			dp[i].clear();
		}
		if (i + 1 == v.size()){
			break;
		}
		for (int j = 0; j <= c; j++){
			if (su[j] == INT_MAX){
				continue;
			}
			dp[j%v[i + 1]].push_back(make_pair(j,su[j]));
		}
		for (int j = 0; j < v[i + 1]; j++){
				sort(dp[j].begin(), dp[j].end());
				for (int k = 1; k < dp[j].size(); k++){
					dp[j][k].second = min(dp[j][k].second, dp[j][k - 1].second + abs(dp[j][k - 1].first - dp[j][k].first) / v[i + 1]);
			}
		}
	}
	int ans = su[c];
	if (ans == INT_MAX){
		puts("-1");
		return 0;
	}
	cout << ans << endl;
	return 0;
}