#include <bits/stdc++.h>
using namespace std;

const int MN = 30, BS = 1<<MN-1;

int N, M, ar[MN], cur[MN], n, s1; bitset<BS> vis; vector<int> dpPv, dpCur, v, ss;

void rec(int i) {
  if (i == n) {
    int rch = 1;
    for (int s: ss)
      rch |= rch << s;
    if (~rch & 1<<s1) return;
    int b = 0;
    for (int j = N; j; j--)
      for (int k = 0; k < cur[j]; k++)
        b <<= j, b |= 1<<j-1;
    //if (n == 2) printf("b %s\n", bitset<MN>(b).to_string().c_str());
    b ^= 1<<N-1;
    if (!vis[b]) dpCur.push_back(b), vis.set(b);
    return;
  }
  for (int j = min(v[i]>>1, s1); j >= 0; j--) {
    ++cur[j]; ++cur[v[i]-j];
    s1 -= j; ss.push_back(v[i]-j - j);
    rec(i+1);
    s1 += j; ss.pop_back();
    --cur[j]; --cur[v[i]-j];
  }
}

int main() {
  ios_base::sync_with_stdio(0); cin.tie(0);

  cin >> N >> M;
  for (int i = 0; i < M; i++) {
    cin >> ar[i];
    ar[i] = min(ar[i], N-ar[i]);
  } sort(ar, ar+M, greater<>());
  vis.set(0);
  dpPv.push_back(0);
  int ans = 0;
  for (int si = 0; si < M; si++) {
    s1 = ar[si];
    for (int b: dpPv) {
      b |= 1<<N-1;
      //if (si == 1) printf("a %s ", bitset<MN>(b).to_string().c_str());
      v.clear();
      while (b) {
        int c = __builtin_ctz(b);
        v.push_back(c+1);
        b >>= c+1;
      } n = v.size();
      /*if (si == 1) {
        printf("%d: ", n);
        for (int a: v) printf("%d ", a);
        printf("\n");
      }*/
      assert(s1 == ar[si]);
      rec(0);
    }
    for (int b: dpCur)
      //printf("%d %s\n", si, bitset<MN>(b|1<<N-1).to_string().c_str()),
      ans = max(ans, __builtin_ctz(~(b|1<<N-1)));
    swap(dpPv, dpCur);
    dpCur.clear();
  }
  printf("%d\n", ans);

  return 0;
}