#include <bits/stdc++.h>
using namespace std;
template <class T>
T sqr(T x) {
  return x * x;
}
template <class T>
T gcd(T a, T b) {
  return (b != 0 ? gcd<T>(b, a % b) : a);
}
template <class T>
T lcm(T a, T b) {
  return (a / gcd<T>(a, b) * b);
}
template <class T>
inline T bigmod(T p, T e, T M) {
  if (e == 0) return 1;
  if (e % 2 == 0) {
    long long int t = bigmod(p, e / 2, M);
    return (T)((t * t) % M);
  }
  return (T)((long long int)bigmod(p, e - 1, M) * (long long int)p) % M;
}
template <class T>
inline T bigexp(T p, T e) {
  if (e == 0) return 1;
  if (e % 2 == 0) {
    long long int t = bigexp(p, e / 2);
    return (T)((t * t));
  }
  return (T)((long long int)bigexp(p, e - 1) * (long long int)p);
}
template <class T>
inline T modinverse(T a, T M) {
  return bigmod(a, M - 2, M);
}
int dx4[] = {1, 0, -1, 0};
int dy4[] = {0, 1, 0, -1};
int dx8[] = {1, 1, 0, -1, -1, -1, 0, 1};
int dy8[] = {0, 1, 1, 1, 0, -1, -1, -1};
int nx8[] = {1, 1, -1, -1, 2, 2, -2, -2};
int ny8[] = {2, -2, 2, -2, 1, -1, 1, -1};
int month[] = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
int n, k, a[1005], dist[2005], tmp, b[1005];
queue<int> q;
int u, v;
int main() {
  scanf("%d %d", &n, &k);
  for (__typeof(k) i = (1); i <= (k); i++) {
    scanf("%d", &tmp);
    b[tmp] = 1;
  }
  int c = 0;
  for (int i = 0; i <= 1000; i++)
    if (b[i] == 1) a[++c] = i - n;
  for (int i = 0; i <= 2000; i++) dist[i] = 1000000007;
  q.push(1000);
  while (!q.empty()) {
    u = q.front();
    q.pop();
    for (int i = 1; i <= c; i++) {
      if (u + a[i] >= 0 && u + a[i] <= 2000) {
        v = u + a[i];
        tmp = dist[u];
        if (tmp == 1000000007) tmp = 0;
        if (dist[v] > tmp + 1) {
          dist[v] = tmp + 1;
          q.push(v);
        }
      }
    }
  }
  if (dist[1000] == 1000000007)
    printf("%d", -1);
  else
    printf("%d", dist[1000]);
  return 0;
}
