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*/

#include <cstdio>
#include <bits/stdc++.h>

using namespace std;

/** Interface */

inline int readChar();
template <class T = int> inline T readInt();
template <class T> inline void writeInt( T x, char end = 0 );
inline void writeChar( int x );
inline void writeWord( const char *s );

/** Read */

static const int buf_size = 4096;

inline int getChar() {
	static char buf[buf_size];
	static int len = 0, pos = 0;
	if (pos == len)
		pos = 0, len = fread(buf, 1, buf_size, stdin);
	if (pos == len)
		return -1;
	return buf[pos++];
}

inline int readChar() {
	int c = getChar();
	while (c <= 32)
		c = getChar();
	return c;
}

template <class T>
inline T readInt() {
	int s = 1, c = readChar();
	T x = 0;
	if (c == '-')
		s = -1, c = getChar();
	while ('0' <= c && c <= '9')
		x = x * 10 + c - '0', c = getChar();
	return s == 1 ? x : -x;
}

/** Write */

static int write_pos = 0;
static char write_buf[buf_size];

inline void writeChar( int x ) {
	if (write_pos == buf_size)
		fwrite(write_buf, 1, buf_size, stdout), write_pos = 0;
	write_buf[write_pos++] = x;
}

template <class T>
inline void writeInt( T x, char end ) {
	if (x < 0)
		writeChar('-'), x = -x;

	char s[24];
	int n = 0;
	while (x || !n)
		s[n++] = '0' + x % 10, x /= 10;
	while (n--)
		writeChar(s[n]);
	if (end)
		writeChar(end);
}

inline void writeWord( const char *s ) {
	while (*s)
		writeChar(*s++);
}

struct Flusher {
	~Flusher() {
		if (write_pos)
			fwrite(write_buf, 1, write_pos, stdout), write_pos = 0;
	}
} flusher;

inline void readWord( char *s ) {
	int c = readChar();
	while (c > 32)
		*s++ = c, c = getChar();
	*s = 0;
}

inline void writeDouble( double x, int output_len ) {
	if (x < 0)
		writeChar('-'), x = -x;
	int t = (int)x;
	writeInt(t), x -= t;
	writeChar('.');
	for (int i = output_len - 1; i > 0; i--) {
		x *= 10;
		t = min(9, (int)x);
		writeChar('0' + t), x -= t;
	}
	x *= 10;
	t = min(9, (int)(x + 0.5));
	writeChar('0' + t);
}








//#pragma comment(linker, "/stack:200000000")
//#pragma GCC optimize("Ofast")
//#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
//#pragma GCC optimize("unroll-loops")

//#define __GLIBCXX_DEBUG

#define shandom_ruffle random_shuffle
#define sz(a) (int)((a).size())
#define all(a) a.begin(), a.end()
#define pb push_back
#define fi first
#define se second
#define mp make_pair
#define x() real()
#define y() imag()
//#define int ll
#define NAME "improvements"

using ll = long long;
using ld = long double;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
using vi = vector<int>;
using vll = vector<ll>;
using ull = unsigned long long;

using pnt = complex<ld>;

istream &operator >> (istream &in, pnt &p) {
    ld a, b;
    in >> a >> b;
    p = {a, b};
    return in;
}

ld crs(pnt a, pnt b) {
    return a.x()*b.y() - a.y()*b.x();
}

ld dot(pnt a, pnt b) {
    return a.x()*b.x() + a.y()*b.y();
}

const int M = 998244353;
const int mod = 1e9+7;
const int inf = 1e9+100;
const ll inf64 = 1e18L;
const ld pi = acos(-1.0L);
const ld eps = 1e-7L;
const int di[] = {0, 1,-1, 0};
const int dj[] = {1, 0, 0,-1};
const int SQ = 1000;
const int LG = 21;
const int nax = 4e5+100; // угадайка

ll l, n, a[nax];
ll p[nax];
ll ans;

ll sum(int l, int r) {
    //assert(0 <= l && r < 2 * n && l <= r);
    return p[r] - (l ? p[l - 1] : 0);
}

main() {
    ios::sync_with_stdio(0), cin.tie(0), cout.tie(0); cout.precision(12); cout << fixed;
#ifdef HOME
    freopen("in.txt", "r", stdin);
#else
    //freopen(NAME".in", "r", stdin); freopen(NAME".out", "w", stdout);
#endif // HOME
    cin >> l >> n;
    for(int i = 0; i < n; i++) cin >> a[i + n], a[i] = l - a[i + n];
    //for(int i = 0; i < 2 * n; i++) cout << a[i] << ' '; cout << endl;
    p[0] = a[0];
    for(int i = 1; i < 2 * n; i++) p[i] = p[i - 1] + a[i];
    ans = max(l - a[n - 1], l - a[n]);
    for(int i = 1; i < n; i++) {
        int j = i + n - 1;
        int li = n - i;
        int lj = i;
        if(li == lj) {
            ans = max(ans, 2 * sum(i, j) - min(a[i], a[j]));
        } else {
            if(li < lj) {
                int id = j - li + 1;
                ans = max({ans, 2 * sum(i, i + li - 1) + 2 * sum(id, j) - min(a[i], a[j]), 2 * sum(i, i + li - 1) + 2 * sum(id - 1, j) - a[j]});
        //        cout << i << ' ' << j << ' ' << 2 * sum(i, i + 2 * li - 2) + 2 * a[j] - a[i] << endl;
                //ans = max({ans, 2 * sum(i, i + 2 * li - 2) + 2 * a[j] - a[i], 2 * sum(i, i + 2 * li - 1) + a[j]});
            } else {
                int id = i + lj - 1;
                ans = max({ans, 2 * sum(i, id) + 2 * sum(j - lj + 1, j) - min(a[i], a[j]), 2 * sum(i, id + 1) + 2 * sum(j - lj + 1, j) - a[i]});
                //ans = max({ans, 2 * sum(j - 2 * lj + 2, j) + 2 * a[i] - a[j], 2 * sum(j - 2 * lj + 1, j) + a[i]});
            }
        }
    }
    cout << ans;
}


Measured memory_bytes: 6528