#include <bits/stdc++.h>
using namespace std;
using lint = long long int;
using pint = pair<int, int>;
using plint = pair<lint, lint>;
struct fast_ios { fast_ios(){ cin.tie(0); ios::sync_with_stdio(false); cout << fixed << setprecision(20); }; } fast_ios_;
#define FOR(i, begin, end) for(int i=(begin),i##_end_=(end);i<i##_end_;i++)
#define IFOR(i, begin, end) for(int i=(end)-1,i##_begin_=(begin);i>=i##_begin_;i--)
#define REP(i, n) FOR(i,0,n)
#define IREP(i, n) IFOR(i,0,n)
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){ os << "["; for (auto v : vec) os << v << ","; os << "]"; return os; }
template<typename T> ostream &operator<<(ostream &os, const deque<T> &vec){ os << "deq["; for (auto v : vec) os << v << ","; os << "]"; return os; }
template<typename T> ostream &operator<<(ostream &os, const set<T> &vec){ os << "{"; for (auto v : vec) os << v << ","; os << "}"; return os; }
template<typename T> ostream &operator<<(ostream &os, const unordered_set<T> &vec){ os << "{"; for (auto v : vec) os << v << ","; os << "}"; return os; }
template<typename T> ostream &operator<<(ostream &os, const multiset<T> &vec){ os << "{"; for (auto v : vec) os << v << ","; os << "}"; return os; }
template<typename T> ostream &operator<<(ostream &os, const unordered_multiset<T> &vec){ os << "{"; for (auto v : vec) os << v << ","; os << "}"; return os; }
template<typename T1, typename T2> ostream &operator<<(ostream &os, const pair<T1, T2> &pa){ os << "(" << pa.first << "," << pa.second << ")"; return os; }
template<typename TK, typename TV> ostream &operator<<(ostream &os, const map<TK, TV> &mp){ os << "{"; for (auto v : mp) os << v.first << "=>" << v.second << ","; os << "}"; return os; }
template<typename TK, typename TV> ostream &operator<<(ostream &os, const unordered_map<TK, TV> &mp){ os << "{"; for (auto v : mp) os << v.first << "=>" << v.second << ","; os << "}"; return os; }
template<typename T> void ndarray(vector<T> &vec, int len) { vec.resize(len); }
template<typename T, typename... Args> void ndarray(vector<T> &vec, int len, Args... args) { vec.resize(len); for (auto &v : vec) ndarray(v, args...); }
template<typename T> void Max(T &m, const T q) { if (m < q) m = q; }
template<typename T> void Min(T &m, const T q) { if (m > q) m = q; }
template<typename T1, typename T2> pair<T1, T2> operator+(pair<T1, T2> &l, pair<T1, T2> &r) { return make_pair(l.first + r.first, l.second + r.second); }
template<typename T1, typename T2> pair<T1, T2> operator-(pair<T1, T2> &l, pair<T1, T2> &r) { return make_pair(l.first - r.first, l.second - r.second); }
#define dbg(x) cerr << #x << " = " << (x) << " (L" << __LINE__ << ") " << __FILE__ << endl;

lint L;
int N;
vector<lint> X;

int main()
{
    cin >> L >> N;
    X.resize(N);
    cin >> X;
    vector<lint> dpl(N + 1), dpr(N + 1);

    REP(i, N)
    {
        dpl[i + 1] = dpl[i] + X[i] * 2;
    }
    REP(i, N)
    {
        dpr[i + 1] = dpr[i] + (L - X[N - 1 - i]) * 2;
    }

    lint ans = max(X.back(), L - X.front());
    FOR(i, 1, N)
    {
        if (i * 2 == N)
        {
            Max(ans, dpl[i] + dpr[i] - X[i - 1]);
            Max(ans, dpl[i] + dpr[i] - L + X[i]);
        }
        else if (i < N - i)
        {
            lint tmp = dpl[i] + dpr[N - i] - dpr[N - i - (i + 1)];
            Max(ans, tmp - L + X[i]);
            tmp = dpl[i] + dpr[N - i] - dpr[N - i - i];
            Max(ans, tmp - X[i - 1]);
        }
        else
        {
            lint tmp = dpr[N - i] + dpl[i] - dpl[i - (N - i) - 1];
            Max(ans, tmp - X[i - 1]);
            tmp = dpr[N - i] + dpl[i] - dpl[i - (N - i)];
            Max(ans, tmp - L + X[i]);
        }
    }
    cout << ans << endl;
}


Measured memory_bytes: 5504