#include<bits/stdc++.h>
// #include<atcoder/all>
// #include<boost/multiprecision/cpp_int.hpp>

using namespace std;
// using namespace atcoder;
// using bint = boost::multiprecision::cpp_int;
using ll = long long;
using ull = unsigned long long;
using ld = long double;
using P = pair<ll,ll>;
using vi = vector<ll>;
using vvi = vector<vi>;
using vvvi = vector<vvi>;
using ve = vector<vector<int>>;
using vb = vector<bool>;
using vvb = vector<vb>;
#define rep(i,n) for(ll i = 0;i < (ll)n;i++)
#define ALL(x) (x).begin(),(x).end()
#define sz(c) ((ll)(c).size())
#define LB(A,x) (int)(lower_bound(A.begin(),A.end(),x)-A.begin())
#define UB(A,x) (int)(upper_bound(A.begin(),A.end(),x)-A.begin())
// #define MOD 1000000007
#define MOD 998244353
template<typename T>using min_priority_queue=priority_queue<T,vector<T>,greater<T>>;
template<typename T>ostream&operator<<(ostream&os,vector<T>&v){for(int i = 0;i < v.size();i++)os<<v[i]<<(i+1!=v.size()?" ":"");return os;}
template<typename T>istream&operator>>(istream&is,vector<T>&v){for(T&in:v)is>>in;return is;}
template<typename T1,typename T2>ostream&operator<<(ostream&os,pair<T1,T2>&p){os<<p.first<<" "<<p.second;return os;}
template<typename T1,typename T2>istream&operator>>(istream&is,pair<T1,T2>&p){is>>p.first>>p.second;return is;}
template<typename T> inline bool chmax(T &a,T b){if(a < b){a = b;return true;}return false;}
template<typename T> inline bool chmin(T &a,T b){if(a > b){a = b;return true;}return false;}
ld dist(ld x1,ld y1,ld x2, ld y2){return sqrtl((x1-x2)*(x1-x2)+(y1-y2)*(y1-y2));}


int main(){
  
  ios_base::sync_with_stdio(0), cin.tie(0);
  int n,K;cin >> n >> K;
  vi a(K);cin >> a;
  rep(i,K)a.emplace_back(a[i]+2*n);
  vi rui(4*n);
  rep(i,K)rui[a[i]]++,rui[a[i]+2*n]++;
  rep(i,4*n-1)rui[i+1] += rui[i];
  ll l = 0,r = 2*n;
  while(r-l > 1){
    static vvi nxt(21,vi(4*n,-1));
    ll mid = (l + r)/2;
    rep(i,4*n){
      if(i & 1){
        if(i+2*mid >= 4*n)nxt[0][i] = -1;
        else if(rui[i+2*mid]-rui[i] > 1){
          nxt[0][i] = -1;
        }else if(rui[i+2*mid]-rui[i] == 1){
          nxt[0][i] = i+2*mid;
        }else{
          int idx = LB(a,i+2*mid);
          if(idx < sz(a))nxt[0][i] = a[idx];
          else nxt[0][i] = -1;
        }
      }
    }
    rep(i,20)rep(j,4*n){
      // if(!(j & 1))break;
      if(nxt[i][j] == -1)nxt[i+1][j] = -1;
      else nxt[i+1][j] = nxt[i][nxt[i][j]];
    }
    bool is = false;
    rep(i,2*n){
      if(!(i & 1))continue;
      int k = i;
      for(int j = 20;j >= 0;j--){
        if(K >> j & 1){
          k = nxt[j][k];
          if(k == -1)break;
        }
      }
      if(k != -1 && k <= i+2*n){
        is = true;break;
      }
    }
    if(is)l = mid;
    else r = mid;
  }
  cout << 2*l << "\n";
  

  return 0;
}