#include<queue>
#include<cstdlib>
#include<cstdio>
using namespace std;

const int MAXN=1000;
const int MAXK=1e6;

int a[MAXK+5];
bool t[MAXN+5];
int n,k;

inline void Init(){
	scanf("%d %d",&n,&k);
	for(int i=1;i<=k;++i)scanf("%d",&a[i]),t[a[i]]=true;
	k=0;int sign=0,flg=1;
	for(int i=1;i<=MAXN;++i)if(t[i]){
		a[++k]=i-n;
		if(a[k]==0){puts("1");exit(0);}
		if((a[k]<0 && sign==1) || (a[k]>0 && sign==-1))flg=0;
		if(a[k]<0)sign=-1;
		else sign=1;
	}
	if(flg){puts("-1");exit(0);}
}

struct node{int w,s;};
queue<node>Q;
bool vis[MAXN*2+5];
inline bool inside(const int x){
	return -MAXN<=x && x<=MAXN;
}
inline void Bfs(){
	for(int i=1;i<=k;++i)Q.push(node{a[i],1}),vis[a[i]+MAXN]=true;
	while(!Q.empty()){
		node now=Q.front();Q.pop();
		int tw;
		for(int i=1;i<=k;++i){
			tw=now.w+a[i];
			if(!inside(tw) || vis[tw+MAXN])continue;
			if(tw==0){
				printf("%d\n",now.s+1);
				return;
			}
			vis[tw+MAXN]=true;
			Q.push(node{tw,now.s+1});
		}
	}
}

int main(){
	Init();
	Bfs();
	return 0;
}