#include <bits/stdc++.h>

#define rep(i, n) for (int i = 0; i < int(n); i++)
#define rrep(i, n) for (int i = int(n) - 1; i >= 0; i--)
#define reps(i, n) for (int i = 1; i <= int(n); i++)
#define rreps(i, n) for (int i = int(n); i >= 1; i--)
#define repc(i, n) for (int i = 0; i <= int(n); i++)
#define rrepc(i, n) for (int i = int(n); i >= 0; i--)
#define repi(i, a, b) for (int i = int(a); i < int(b); i++)
#define repic(i, a, b) for (int i = int(a); i <= int(b); i++)
#define each(x, y) for (auto &x : y)
#define all(a) (a).begin(), (a).end()
#define bit(b) (1ll << (b))

using namespace std;

using i32 = int;
using i64 = long long;
using u64 = unsigned long long;
using f80 = long double;
using vi32 = vector<i32>;
using vi64 = vector<i64>;
using vu64 = vector<u64>;
using vf80 = vector<f80>;
using vstr = vector<string>;

inline void yes() { cout << "Yes" << '\n'; exit(0); }
inline void no() { cout << "No" << '\n'; exit(0); }
inline i64 gcd(i64 a, i64 b) { if (min(a, b) == 0) return max(a, b); if (a % b == 0) return b; return gcd(b, a % b); }
inline i64 lcm(i64 a, i64 b) { return a / gcd(a, b) * b; }
inline u64 xorshift() { static u64 x = 88172645463325252ull; x = x ^ (x << 7); return x = x ^ (x >> 9); }
void solve(); int main() { ios::sync_with_stdio(0); cin.tie(0); cout << fixed << setprecision(16); solve(); return 0; }
template <typename T> class pqasc : public priority_queue<T, vector<T>, greater<T>> {};
template <typename T> class pqdesc : public priority_queue<T, vector<T>, less<T>> {};
template <typename T> inline void amax(T &x, T y) { if (x < y) x = y; }
template <typename T> inline void amin(T &x, T y) { if (x > y) x = y; }
template <typename T> inline T power(T x, i64 n, T e = 1) { T r = e; while (n > 0) { if (n & 1) r *= x; x *= x; n >>= 1; } return r; }
template <typename T> istream& operator>>(istream &is, vector<T> &v) { each(x, v) is >> x; return is; }
template <typename T> ostream& operator<<(ostream &os, vector<T> &v) { rep(i, v.size()) { if (i) os << ' '; os << v[i]; } return os; }
template <typename T, typename S> istream& operator>>(istream &is, pair<T, S> &p) { is >> p.first >> p.second; return is; }
template <typename T, typename S> ostream& operator<<(ostream &os, pair<T, S> &p) { os << p.first << ' ' << p.second; return os; }

void solve() {
  int N, L; cin >> L >> N;
  vector<long long> X(N + 1, 0);
  vector<long long> Y(N + 1, 0);
  for (int i = 1; i <= N; i++) {
    cin >> X[i];
  }
  for (int i = 1; i <= N; i++) {
    Y[i] = L - X[N - i + 1];
  }
  for (int i = 1; i <= N; i++) {
    X[i] += X[i - 1];
    Y[i] += Y[i - 1];
  }
  long long ans = 0;
  for (int i = 1; i <= N; i++) {
    long long sum = 0;
    if ((N - i) % 2 == 0) {
      sum = 2ll * Y[(N - i) / 2] + 2ll * (X[(N + i) / 2] - X[i - 1]) - (X[(N + i) / 2] - X[(N + i) / 2 - 1]);
    } else {
      sum = 2ll * (X[(N + i - 1) / 2] - X[i - 1]) + 2ll * Y[(N - i + 1) / 2] - (Y[(N - i + 1) / 2] - Y[(N - i + 1) / 2 - 1]);
    }
    ans = max(ans, sum);
  }
  for (int i = 1; i <= N; i++) {
    long long sum = 0;
    if ((N - i) % 2 == 0) {
      sum = 2ll * X[(N - i) / 2] + 2ll * (Y[(N + i) / 2] - Y[i - 1]) - (Y[(N + i) / 2] - Y[(N + i) / 2 - 1]);
    } else {
      sum = 2ll * (Y[(N + i - 1) / 2] - Y[i - 1]) + 2ll * X[(N - i + 1) / 2] - (X[(N - i + 1) / 2] - X[(N - i + 1) / 2 - 1]);
    }
    ans = max(ans, sum);
  }
  cout << ans << '\n';
}


Measured memory_bytes: 3328