#include <bits/stdc++.h>
using namespace std;
int n, m;
int dp[3333];
bool d[3333];
int a[3333];
bool f[3333];
int qu[3333];
int main() {
  ios::sync_with_stdio(false);
  cin >> n >> m;
  for (int i = 1; i <= m; i++) {
    int x;
    cin >> x;
    d[x] = true;
  }
  m = 0;
  for (int i = 0; i <= 1000; i++)
    if (d[i]) {
      a[++m] = i - n;
    }
  qu[1] = 0;
  f[1000] = true;
  int l = 1;
  int r = 1;
  while (l <= r) {
    int u = qu[l];
    l++;
    for (int i = 1; i <= m; i++) {
      int v = u + a[i];
      if (v >= -1000 && v <= 1000) {
        if (f[v + 1000] == false) {
          f[v + 1000] = true;
          qu[++r] = v;
          dp[v + 1000] = dp[u + 1000] + 1;
        }
      }
    }
  }
  int ret = 1000000001;
  for (int i = -1000; i <= 1000; i++)
    if (f[i + 1000]) {
      bool ok = false;
      for (int j = 1; j <= m; j++)
        if (i + a[j] == 0) {
          ok = true;
          break;
        }
      if (ok) ret = min(ret, dp[i + 1000] + 1);
    }
  if (ret == 1000000001)
    cout << -1;
  else
    cout << ret;
  return 0;
}
