#include <bits/stdc++.h>
using namespace std;
int a[1000010], b[1000010];
int f[1000010], g[1000010];
int tmp[1000010];
int ansf[1000010], ansg[1000010];
int main() {
  int sv, n, m;
  scanf("%d %d", &sv, &n);
  int p = 0, q = 0;
  for (int i = 1; i <= n; i++) {
    int x;
    scanf("%d", &x);
    x -= sv;
    if (x == 0) {
      puts("1");
      return 0;
    }
    int y = abs(x);
    if (x < 0)
      a[++p] = y;
    else
      b[++q] = y;
  }
  if (!p || !q) {
    puts("-1");
    return 0;
  }
  sort(a + 1, a + p + 1);
  p = unique(a + 1, a + p + 1) - a - 1;
  sort(b + 1, b + q + 1);
  q = unique(b + 1, b + q + 1) - b - 1;
  n = 0;
  for (int i = 1; i <= p; i++) {
    f[++n] = a[i];
    ansf[a[i]] = 1;
  }
  for (int i = 1; i < 1005; i++) {
    int N = 0;
    for (int j = 1; j <= n; j++) {
      for (int k = 1; k <= p; k++) {
        int x = f[j] + a[k];
        if (!ansf[x]) {
          ansf[x] = i + 1;
          tmp[++N] = x;
        }
      }
    }
    n = N;
    for (int j = 1; j <= n; j++) f[j] = tmp[j];
  }
  m = 0;
  for (int i = 1; i <= q; i++) {
    g[++m] = b[i];
    ansg[b[i]] = 1;
  }
  for (int i = 1; i < 1005; i++) {
    int N = 0;
    for (int j = 1; j <= m; j++) {
      for (int k = 1; k <= q; k++) {
        int x = g[j] + b[k];
        if (!ansg[x]) {
          ansg[x] = i + 1;
          tmp[++N] = x;
        }
      }
    }
    m = N;
    for (int j = 1; j <= m; j++) g[j] = tmp[j];
  }
  int res = 0x3f3f3f3f;
  for (int i = 1; i < 1000010; i++) {
    if (ansf[i] && ansg[i]) res = min(res, ansf[i] + ansg[i]);
  }
  cout << res << endl;
}
