#include <bits/stdc++.h>

#define N 3006

#define setIO(s) freopen(s".in","r",stdin)

using namespace std;

int n,k,vis[N],in[N],dp[N];

vector<int>v;

queue<int>q;

int main()

{

    int i,j;

    // setIO("input");

    scanf("%d%d",&n,&k);

    for(i=1;i<=k;++i)

    {

        int x;

        scanf("%d",&x), in[x]=1;

    }

    memset(dp,0x3f,sizeof(dp));

    for(i=0;i<=1000;++i)

    {

        if(in[i])

        {

            v.push_back(n-i);

            int x=n-i+1000;

            dp[x]=1;

            if(x==1000)

            {

               printf("1\n");

               return 0;

            }

            q.push(x);

        }

    }

    for(;!q.empty();)

    {

        int u=q.front();q.pop();

        for(i=0;i<v.size();++i)

        {

            int x=u+v[i];

            if(x>=0&&x<=2000&&dp[x]>dp[u]+1)

            {

                dp[x]=dp[u]+1;

                q.push(x);

            }

        }

    }

    if(dp[1000]>=100000) printf("-1\n");

    else printf("%d\n",dp[1000]);

    return 0;

}