#include <algorithm>
#include <bitset>
#include <cmath>
#include <cstring>
#include <iomanip>
#include <iostream>
#include <map>
#include <numeric>
#include <queue>
#include <regex>
#include <set>
#include <stack>
#include <string>
#include <vector>

const int MOD = 1e9 + 7;
const int iINF = 2147483647 / 2;
const long long int llINF = 9223372036854775807 / 2;

using namespace std;
using ll = long long int;
using vl = vector<ll>;
using vvl = vector<vector<ll>>;
using vvvl = vector<vector<vector<ll>>>;

typedef pair<ll, ll> pll;
bool paircomp(const pll &a, const pll &b) {
  if (a.first == b.first)
    return a.second < b.second;
  return a.first < b.first;
}
struct multi {
  ll first;
  ll second;
  ll third;
  multi(ll first, ll second, ll third)
      : first(first), second(second), third(third) {}
};
bool multicomp(const multi &a, const multi &b) {
  if (a.first == b.first)
    return a.second < b.second;
  return a.first < b.first;
}
#define REP(i, n) for (ll i = 0; i < (n); i++)
#define RREP(i, n) for (ll i = (n)-1; i >= 0; i--)
#define FOR(i, a, b) for (ll i = (a); i < (b); i++)
#define AUTO(i, m) for (auto &i : m)
#define ALL(a) (a).begin(), (a).end()
#define MAX(vec) *std::max_element(vec.begin(), vec.end())
#define MIN(vec) *std::min_element(vec.begin(), vec.end())
#define ARGMAX(vec)                                                            \
  std::distance(vec.begin(), std::max_element(vec.begin(), vec.end()))
#define ARGMIN(vec)                                                            \
  std::distance(vec.begin(), std::min_element(vec.begin(), vec.end()))
#define REV(T) greater<T>()
#define PQ(T) priority_queue<T, vector<T>, greater<T>>
#define VVL(a, b, c) vector<vector<ll>>(a, vector<ll>(b, c))
#define VVVL(a, b, c, d)                                                       \
  vector<vector<vector<ll>>>(a, vector<vector<ll>>(b, vector<ll>(c, d)))
#define SP(a) setprecision(a)
#define SQRT(a) sqrt((long double)(a))
#define DPOW(a, b) pow((long double)(a), (long double)(b))
#define UNIQUE(vec)                                                            \
  do {                                                                         \
    sort(ALL((vec)));                                                          \
    (vec).erase(std::unique(ALL((vec))), (vec).end());                         \
  } while (0)

ll POW(ll n, ll m) {
  if (m == 0) {
    return 1;
  } else if (m % 2 == 0) {
    ll tmp = POW(n, m / 2);
    return (tmp * tmp);
  } else {
    return (n * POW(n, m - 1));
  }
}

int dx[4] = {1, 0, -1, 0};
int dy[4] = {0, 1, 0, -1};

int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  ll L, N;
  cin >> L >> N;
  vl X(N);
  vl XR(N);
  vl DP(N + 1, 0);
  vl DPR(N + 1, 0);
  ll ans = 0;
  REP(i, N) {
    cin >> X[i];
    XR[N - 1 - i] = L - X[i];
  }
  REP(i, N) {
    DP[i + 1] = DP[i] + X[i];
    DPR[i + 1] = DPR[i] + XR[i];
  }
  FOR(i, 1, N + 1) {
    ll score = 2 * (DPR[((N - i) / 2) + i] - DPR[i - 1]);
    score += 2 * (DP[((N - i) / 2) + ((N - i) % 2)]);
    if ((N - i) % 2 == 1)
      score -= X[((N - i) / 2) + ((N - i) % 2) - 1];
    else
      score -= XR[((N - i) / 2) + i - 1];
    ans = max(ans, score);
  }
  swap(X, XR);
  swap(DP, DPR);
  FOR(i, 1, N + 1) {
    ll score = 2 * (DPR[((N - i) / 2) + i] - DPR[i - 1]);
    score += 2 * (DP[((N - i) / 2) + ((N - i) % 2)]);
    if ((N - i) % 2 == 1)
      score -= X[((N - i) / 2) + ((N - i) % 2) - 1];
    else
      score -= XR[((N - i) / 2) + i - 1];
    ans = max(ans, score);
  }

  cout << ans << endl;
  return 0;
}


Measured memory_bytes: 6528