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

// macros
#define pb emplace_back
#define mk make_pair
#define FOR(i, a, b) for(int i=(a);i<(b);++i)
#define rep(i, n) FOR(i, 0, n)
#define rrep(i, n) for(int i=((int)(n)-1);i>=0;i--)
#define irep(itr, st) for(auto itr = (st).begin(); itr != (st).end(); ++itr)
#define irrep(itr, st) for(auto itr = (st).rbegin(); itr != (st).rend(); ++itr)
#define whole(x) (x).begin(),(x).end()
#define sz(x) ((int)(x).size())
#define bit(n) (1LL<<(n))
#define F first
#define S second
// functions
template <typename T> void unique(T& c){c.erase(std::unique(c.begin(), c.end()), c.end());}
template <class T>bool chmax(T &a, const T &b){if(a < b){a = b; return 1;} return 0;}
template <class T>bool chmin(T &a, const T &b){if(a > b){a = b; return 1;} return 0;}
template <typename T> istream &operator>>(istream &is, vector<T> &vec){for(auto &v: vec)is >> v; return is;}
template <typename T> ostream &operator<<(ostream &os, const vector<T>& vec){for(int i = 0; i < vec.size(); i++){ os << vec[i]; if(i + 1 != vec.size())os << " ";} return os;}
template <typename T> ostream &operator<<(ostream &os, const set<T>& st){for(auto itr = st.begin(); itr != st.end(); ++itr){ os << *itr; auto titr = itr; if(++titr != st.end())os << " ";} return os;}
template <typename T> ostream &operator<<(ostream &os, const unordered_set<T>& st){for(auto itr = st.begin(); itr != st.end(); ++itr){ os << *itr; auto titr = itr; if(++titr != st.end())os << " ";} return os;}
template <typename T> ostream &operator<<(ostream &os, const multiset<T>& st){for(auto itr = st.begin(); itr != st.end(); ++itr){ os << *itr; auto titr = itr; if(++titr != st.end())os << " ";} return os;}
template <typename T> ostream &operator<<(ostream &os, const unordered_multiset<T>& st){for(auto itr = st.begin(); itr != st.end(); ++itr){ os << *itr; auto titr = itr; if(++titr != st.end())os << " ";} return os;}
template <typename T1, typename T2> ostream &operator<<(ostream &os, const pair<T1, T2> &p){os << "(" << p.first << ", " << p.second << ")"; return os;}
template <typename T1, typename T2> ostream &operator<<(ostream &os, const map<T1, T2> &mp){for(auto itr = mp.begin(); itr != mp.end(); ++itr){ os << "(" << itr->first << ", " << itr->second << ")"; auto titr = itr; if(++titr != mp.end())os << " "; } return os;}
template <typename T1, typename T2> ostream &operator<<(ostream &os, const unordered_map<T1, T2> &mp){for(auto itr = mp.begin(); itr != mp.end(); ++itr){ os << "(" << itr->first << ", " << itr->second << ")"; auto titr = itr; if(++titr != mp.end())os << " "; } return os;}
//  types
using ll = long long int;
using P = pair<int, int>;
// constants
const int inf = 1e9;
const ll linf = 1LL << 60;
const double EPS = 1e-10;
const int mod = 1000000007;
const int dx[4] = {-1, 0, 1, 0};
const int dy[4] = {0, -1, 0, 1};
// io
struct fast_io{
  fast_io(){ios_base::sync_with_stdio(false); cin.tie(nullptr); cout << fixed << setprecision(20);}
} fast_io_;

ll solve(ll l, int n, vector<ll> &x){
  ll res = 0;
  vector<ll> acc = x; FOR(i, 1, n)acc[i] += acc[i-1];
  vector<ll> iacc(n);
  rep(i, n){
    iacc[i] = l - x[i];
    if(i > 0)iacc[i] += iacc[i-1];
  }
  int mid = (n + 1) / 2;
  ll tmp = acc[mid-1] * 2 + (iacc[n-1] - iacc[mid-1]) * 2;
  if(n % 2 == 0)tmp -= (l - x[mid]);
  else tmp -= x[mid-1];
  chmax(res, tmp);
  chmax(res, x.back());

  for(int i = 1; i < n; i++){
    tmp = 0;
    mid = (n - i + 1) / 2;
    tmp += (acc[i+mid-1] - acc[i-1]) * 2;
    tmp += (iacc[n-1] - iacc[i+mid-1]) * 2;
    if((n - i) % 2 == 0)tmp -= l - x[i+mid];
    else tmp -= x[i+mid-1];
    chmax(res, tmp);
  }
  return res;
}

int main(int argc, char const* argv[])
{
  ll l, n; cin >> l >> n;
  vector<ll> x(n); cin >> x;
  if(n == 1){
    cout << max(x[0], l - x[0]) << endl;
    return 0;
  }
  ll res = 0;
  chmax(res, solve(l, n, x));
  rep(i, n)x[i] = l - x[i];
  reverse(whole(x));
  chmax(res, solve(l, n, x));
  cout << res << endl;
  return 0;
}


Measured memory_bytes: 7944