/* #region header */
#ifdef LOCAL
    #include "/Users/takakurashokichi/Desktop/atcoder/cxx-prettyprint-master/prettyprint.hpp"
    #define debug(x) cout << x << endl
#else
    #define debug(...) 42
#endif

#pragma GCC optimize("Ofast")
#include <bits/stdc++.h>
//types
using namespace std;
using ll = long long;
using ull = unsigned long long;
using ld = long double;
typedef pair < ll , ll > Pl;        
typedef pair < int, int > Pi;
typedef vector<ll> vl;
typedef vector<int> vi;
typedef vector<char> vc;
template< typename T >
using mat = vector< vector< T > >;
typedef vector<vector<int>> vvi;
typedef vector<vector<long long>> vvl;
typedef vector<vector<char>> vvc;

template< int mod >
struct modint {
    int x;

    modint() : x(0) {}

    modint(int64_t y) : x(y >= 0 ? y % mod : (mod - (-y) % mod) % mod) {}

    modint &operator+=(const modint &p) {
        if((x += p.x) >= mod) x -= mod;
        return *this;
    }

    modint &operator-=(const modint &p) {
        if((x += mod - p.x) >= mod) x -= mod;
        return *this;
    }

    modint &operator*=(const modint &p) {
        x = (int) (1LL * x * p.x % mod);
        return *this;
    }

    modint &operator/=(const modint &p) {
        *this *= p.inverse();
        return *this;
    }

    modint operator-() const { return modint(-x); }

    modint operator+(const modint &p) const { return modint(*this) += p; }

    modint operator-(const modint &p) const { return modint(*this) -= p; }

    modint operator*(const modint &p) const { return modint(*this) *= p; }

    modint operator/(const modint &p) const { return modint(*this) /= p; }

    bool operator==(const modint &p) const { return x == p.x; }

    bool operator!=(const modint &p) const { return x != p.x; }

    modint inverse() const {
        int a = x, b = mod, u = 1, v = 0, t;
        while(b > 0) {
        t = a / b;
        swap(a -= t * b, b);
        swap(u -= t * v, v);
        }
        return modint(u);
    }

    modint pow(int64_t n) const {
        modint ret(1), mul(x);
        while(n > 0) {
        if(n & 1) ret *= mul;
        mul *= mul;
        n >>= 1;
        }
        return ret;
    }

    friend ostream &operator<<(ostream &os, const modint &p) {
        return os << p.x;
    }

    friend istream &operator>>(istream &is, modint &a) {
        int64_t t;
        is >> t;
        a = modint< mod >(t);
        return (is);
    }

    static int get_mod() { return mod; }
};
//abreviations
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
#define rep_(i, a_, b_, a, b, ...) for (ll i = (a), max_i = (b); i < max_i; i++)
#define rep(i, ...) rep_(i, __VA_ARGS__, __VA_ARGS__, 0, __VA_ARGS__)
#define rrep_(i, a_, b_, a, b, ...) for (ll i = (b-1), min_i = (a); i >= min_i; i--)
#define rrep(i, ...) rrep_(i, __VA_ARGS__, __VA_ARGS__, 0, __VA_ARGS__)
#define SZ(x) ((int)(x).size())
#define pb(x) push_back(x)
#define eb(x) emplace_back(x)
#define mp make_pair
#define print(x) cout << x << endl
#define vprint(x) rep(i, x.size())cout << x[i] << ' '; cout << endl
#define vsum(x) accumulate(all(x), 0LL)
#define vmax(a) *max_element(all(a))
#define vmin(a) *min_element(all(a))
#define lb(c, x) distance((c).begin(), lower_bound(all(c), (x)))
#define ub(c, x) distance((c).begin(), upper_bound(all(c), (x)))
//functions
//gcd(0, x) fails.
ll gcd(ll a, ll b) { return b ? gcd(b, a%b) : a; }
ll lcm(ll a, ll b) { return a/gcd(a, b)*b;}
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 (b<a) { a=b; return 1; } return 0; }
template< typename T >
T mypow(T x, ll n) {
    T ret = 1;
    while(n > 0) {
        if(n & 1) (ret *= x);
        (x *= x);
        n >>= 1;
    }
    return ret;
}
ll modpow(ll x, ll n, const ll mod) {
    ll ret = 1;
    while(n > 0) {
        if(n & 1) (ret *= x);
        (x *= x);
        n >>= 1;
        x%=mod;
        ret%=mod;
    }
    return ret;
}
template< typename T >
uint64_t my_rand(void) {
    static uint64_t x = 88172645463325252ULL;
    x = x ^ (x << 13); x = x ^ (x >> 7);
    return x = x ^ (x << 17);
}
int popcnt(ull x) { return __builtin_popcountll(x); } 
//graph template
template< typename T >
struct edge {
    int src, to;
    T cost;

    edge(int to, T cost) : src(-1), to(to), cost(cost) {}

    edge(int src, int to, T cost) : src(src), to(to), cost(cost) {}

    edge &operator=(const int &x) {
        to = x;
        return *this;
    }

    bool operator<(const edge<T> &r) const { return cost < r.cost; }
    
    operator int() const { return to; }
};
template< typename T >
using Edges = vector< edge< T > >;
template< typename T >
using WeightedGraph = vector< Edges< T > >;
using UnWeightedGraph = vector< vector< int > >;
struct Timer { 
    clock_t start_time; 
    void start() {
        start_time = clock(); 
    }
    int lap() { 
        //return x ms.
        return (clock()-start_time)*1000 / CLOCKS_PER_SEC; 
    }
};
/* #endregion*/
//constant
#define inf 1000000005
#define INF 4000000004000000000LL
#define mod 998244353LL
//#define endl '\n'
typedef modint<mod> mint;
const long double eps = 0.00000001;
const long double PI  = 3.141592653589793;
//library
ll solve(int n, ll l, vl x){
    ll finalAns = 0;
    ll ans = x[0];
    rep(i, 1, n){
        if(i%2==0)ans+=(l-x[n-(i+1)/2])+x[(i)/2];
        else ans+=x[i/2]+(l-x[n-(i+1)/2]);
    }
    finalAns = ans;

    rep(i, 1, n){
        if((n-i)%2){
            int r = (n-i-1)/2;
            ans+=(2*(x[i+r]-x[i-1])-l);
        }else{
            int r = (n-i-2)/2;
            ans+=(2*((x[i+r]-x[i-1]))-(l-x[i+r+1]+x[i+r]));
        }
        chmax(finalAns, ans);
    }

    return finalAns;
}
int main(){
    cin.tie(0);
    ios::sync_with_stdio(0);
    cout << setprecision(20);
    ll l, n; cin>>l>>n;
    vl x(n);
    rep(i, n)cin>>x[i];
    ll ans = solve(n, l, x);
    reverse(all(x));
    rep(i, n)x[i] = l-x[i];
    chmax(ans, solve(n, l, x));
    print(ans);
}

Measured memory_bytes: 6372