#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)

void solve() {
  ll n, k;
  cin >> n >> k;
  vector<ll> a(k);
  rep(i, k) cin >> a[i];
  {
    vector<ll> na(k);
    ll min_d = 1e18, min_i = -1;
    rep(i, k) {
      ll d = a[(i + 1) % k] - a[i];
      if (i == k - 1)
        d += n * 2;
      if (d < min_d) {
        min_d = d;
        min_i = i;
      }
    }
    ll x = a[min_i] - 1;
    rep(i, k) {
      ll j = (i + min_i) % k;
      na[i] = a[j] - x;
      if (j < min_i)
        na[i] += n * 2;
    }
    swap(a, na);
  }
  vector<ll> sum(n * 2 + 10, 0);
  {
    ll j = 0;
    rep(i, n * 2 + 5) {
      sum[i + 1] = sum[i];
      if (j < k && a[j] == i + 1) {
        sum[i + 1]++;
        j++;
      }
    }
  }
  ll ans = -1;
  for (ll s = 2; s < a[1]; s += 2) {
    ll L = 2, R = n + 1;
    while (L + 1 < R) {
      ll w = (L + R) / 2, now = s;
      bool ok = true;
      rep(i, k - 1) {
        ll nex = max(now + w, a[i + 1] + 1);
        if (nex > n * 2) {
          ok = false;
          break;
        }
        ok &= sum[nex] - sum[now] == 1;
        if (!ok)
          break;
        now = nex;
      }
      ok &= sum[n * 2 + 1] - sum[now] == 0;
      ok &= n * 2 - now + s >= w;
      if (ok) {
        L = w;
      } else {
        R = w;
      }
    }
    ans = max(ans, L);
  }
  if (ans % 2)
    ans--;
  cout << ans << '\n';
}

int main() {
  std::cin.tie(nullptr);
  std::ios_base::sync_with_stdio(false);
  int T = 1;
  for (int t = 0; t < T; t++) {
    solve();
  }
  return 0;
}