#include<bits/stdc++.h>

#define LL long long

#define fi first

#define se second

#define mk make_pair

#define PLL pair<LL, LL>

#define PLI pair<LL, int>

#define PII pair<int, int>

#define SZ(x) ((int)x.size())

#define ull unsigned long long

using namespace std;

const int N =  + ;

const int inf = 0x3f3f3f3f;

const LL INF = 0x3f3f3f3f3f3f3f3f;

const int mod = 1e9 + ;

const double eps = 1e-;

const double PI = acos(-);

int n, k, a[N];

bitset<> dp[N];

int main() {

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

    for(int i = ; i <= k; i++) scanf("%d", &a[i]);

    for(int i = ; i <= k; i++) {

        if(a[i] == n) {

            puts("");

            return ;

        }

        a[i] -= n;

    }

    sort(a + , a +  + k);

    k = unique(a + , a +  + k) - a - ;

    dp[][] = ;

    for(int i = ; i <= ; i++) {

        for(int j = ; j <= k; j++) {

            if(a[j] >= ) dp[i] |= dp[i - ] << a[j];

            else dp[i] |= dp[i - ] >> (-a[j]);

            if(dp[i][]) {

                printf("%d\n", i);

                return ;

            }

        }

    }

    puts("-1");

    return ;

}

/*

*/