///////////////////////////////////////////////////////////////////////////////

#include <bits/stdc++.h>
#include <unistd.h>
#include <time.h>
#include <sys/time.h>

using namespace std;

///////////////////////////////////////////////////////////////////////////////

#define DEBUG 0

#define pb push_back
#define V vector
#define M unordered_map
#define S static

#define rep(i,n) for(ll i=0LL; i<n; ++i)
#define srep(i,s,n) for(ll i=s; i<n; ++i)
#define rrep(i,n) for(ll i=n-1LL; i>=0LL; --i)
#define ALL(a) (a).begin(),(a).end()
#define RALL(a) (a).rbegin(),(a).rend()
#define CIN(x) do { \
        assert(!cin.eof()); \
        cin >> x; \
        assert(!cin.fail()); \
} while(0);
#if DEBUG
#define debug_print(...) _debug_print(__VA_ARGS__)
#define debug_printf(...) printf(__VA_ARGS__)
#define debug_print_time _debug_print_time
#else // DEBUG
#define debug_print(...)
#define debug_printf(...)
#define debug_print_time
#endif // DEBUG

typedef long long ll;
typedef unsigned long long ull;
typedef tuple<ll, ll> t2;
typedef tuple<ll, ll, ll> t3;
typedef tuple<ll, ll, ll, ll> t4;
typedef tuple<ll, ll, ll, ll, ll> t5;
template <typename T>
using priority_queue_incr = priority_queue<T, V<T>, greater<T> >;

///////////////////////////////////////////////////////////////////////////////

template <typename TT, typename T>
T get_m(M<TT, T> &m, TT k, T default_value)
{
    if (m.find(k) == m.end()) return m[k] = default_value;
    return m[k];
}

template <typename TT, typename T>
void incr_m(M<TT, T> &m, TT k)
{
    if (m.find(k) == m.end()) m[k] = 0;
    m[k]++;
}

template <typename TT, typename T>
void incr_m(map<TT, T> &m, TT k)
{
    if (m.find(k) == m.end()) m[k] = 0;
    m[k]++;
}

void llin(ll &a)
{
    CIN(a);
}

void llinl1(V<ll> &v, ll count)
{
    for (ll i = 0LL; i < count; ++i) {
        ll a;
        CIN(a);
        v.push_back(a);
    }
}

void llinl2(V<t2> &v, ll count)
{
    for (ll i = 0LL; i < count; ++i) {
        ll a, b;
        CIN(a >> b);
        v.push_back(t2(a, b));
    }
}

void llinl3(V<t3> &v, ll count)
{
    for (ll i = 0LL; i < count; ++i) {
        ll a, b, c;
        CIN(a >> b >> c);
        v.push_back(t3(a, b, c));
    }
}

void llinl4(V<t4> &v, ll count)
{
    for (ll i = 0LL; i < count; ++i) {
        ll a, b, c, d;
        CIN(a >> b >> c >> d);
        v.push_back(t4(a, b, c, d));
    }
}

void llina(V<ll> &v, ll count)
{
    llinl1(v, count);
}

template <typename T>
void sort(V<T> &v)
{
    sort(v.begin(), v.end());
}

template <typename T>
void sort_reverse(V<T> &v)
{
    sort(v.begin(), v.end(), greater<T>());
}

template <typename T>
void _debug_print(T x)
{
    cout << x << endl;
}

void _debug_print(const t2 &x)
{
    ll x1 = get<0>(x);
    ll x2 = get<1>(x);
    cout << "-- " << x1 << " -- " << x2 << endl;
}

void _debug_print(const t3 &x)
{
    ll x1 = get<0>(x);
    ll x2 = get<1>(x);
    ll x3 = get<2>(x);
    cout << "-- " << x1 << " -- " << x2 << " -- " << x3 << endl;
}

void _debug_print(const t4 &x)
{
    ll x1 = get<0>(x);
    ll x2 = get<1>(x);
    ll x3 = get<2>(x);
    ll x4 = get<3>(x);
    cout << "-- " << x1 << " -- " << x2
         << " -- " << x3 << " -- " << x4 << endl;
}

template <typename T>
void _debug_print(T xarray[], ll n)
{
    rep (i, n) _debug_print(xarray[i]);
}

template <typename T>
void _debug_print(const V<T> &xlist)
{
    for (auto x : xlist) {
        cout << "-- ";
        _debug_print(x);
    }
}

template <typename T>
void _debug_print(const set<T> &xset)
{
    for (auto x : xset) {
        cout << "-- ";
        _debug_print(x);
    }
}

template <typename TT, typename T>
void _debug_print(const M<TT, T> &xlist)
{
    for (auto x : xlist) {
        TT k = x.first;
        T v = x.second;
        cout << "====" << endl;
        cout << "K=";
        _debug_print(k);
        cout << "V=";
        _debug_print(v);
    }
}

template <typename TT, typename T>
void _debug_print(const map<TT, T> &xlist)
{
    for (auto x : xlist) {
        TT k = x.first;
        T v = x.second;
        cout << "====" << endl;
        cout << "K=";
        _debug_print(k);
        cout << "V=";
        _debug_print(v);
    }
}

void _debug_print_time(const char *prefix)
{
    struct timeval tv;
    gettimeofday(&tv, NULL);
#if DEBUG
    struct tm *tm = localtime(&tv.tv_sec);
#endif
    debug_printf("-- %s %02d:%02d:%02d.%06ld\n",
                 prefix, tm->tm_hour, tm->tm_min, tm->tm_sec, tv.tv_usec);
}

///////////////////////////////////////////////////////////////////////////////

void _main();
int main()
{
    cout << setprecision(12);
#if !DEBUG
    ios::sync_with_stdio(false);
    cin.tie(0);
#endif

    _main();
    return 0;
}

void _main()
{
    ll total; llin(total);
    ll n; llin(n);
    V<ll> xlist; llina(xlist, n);

    V<ll> rxlist;
    rrep (i, n) rxlist.pb(total - xlist[i]);

    debug_printf("---- xlist\n");
    debug_print(xlist);
    debug_printf("---- rxlist\n");
    debug_print(rxlist);

    V<ll> sumlist;
    ll sum = 0;
    sumlist.pb(sum);
    for (auto x : xlist) {
        sum += x;
        sumlist.pb(sum);
    }

    V<ll> rsumlist;
    ll rsum = 0;
    rsumlist.pb(rsum);
    for (auto x : rxlist) {
        rsum += x;
        rsumlist.pb(rsum);
    }

    rep (i, n+1) sumlist[i] *= 2LL;
    rep (i, n+1) rsumlist[i] *= 2LL;

    debug_printf("---- sumlist\n");
    debug_print(sumlist);
    debug_printf("---- rsumlist\n");
    debug_print(rsumlist);

    ll ans = 0;
    ans = max(ans, total - xlist[0]);
    ans = max(ans, total - rxlist[0]);

    ll cnt = n-1;
    ll rcnt = 1;
    while (cnt-rcnt-1 >= 0) {
        ll lsum = sumlist[cnt] - sumlist[cnt-rcnt-1];
        lsum += rsumlist[rcnt];
        lsum -= xlist[cnt-1];
        debug_printf("xx1 cnt=%lld rcnt=%lld lsum=%lld\n", cnt, rcnt, lsum);
        ans = max(ans, lsum);
        cnt--;
        rcnt++;
    }

    cnt = 1;
    rcnt = n-1;
    while (rcnt-cnt-1 >= 0) {
        ll lsum = rsumlist[rcnt] - rsumlist[rcnt-cnt-1];
        lsum += sumlist[cnt];
        lsum -= rxlist[rcnt-1];
        debug_printf("xx2 cnt=%lld rcnt=%lld lsum=%lld\n", cnt, rcnt, lsum);
        ans = max(ans, lsum);
        cnt++;
        rcnt--;
    }

    cnt = n-1;
    rcnt = 1;
    while (cnt-rcnt >= 0) {
        ll lsum = sumlist[cnt] - sumlist[cnt-rcnt];
        lsum += rsumlist[rcnt];
        lsum -= rxlist[rcnt-1];
        debug_printf("xx3 cnt=%lld rcnt=%lld lsum=%lld\n", cnt, rcnt, lsum);
        ans = max(ans, lsum);
        cnt--;
        rcnt++;
    }

    cnt = 1;
    rcnt = n-1;
    while (rcnt-cnt >= 0) {
        ll lsum = rsumlist[rcnt] - rsumlist[rcnt-cnt];
        lsum += sumlist[cnt];
        lsum -= xlist[cnt-1];
        debug_printf("xx4 cnt=%lld rcnt=%lld lsum=%lld\n", cnt, rcnt, lsum);
        ans = max(ans, lsum);
        cnt++;
        rcnt--;
    }

    debug_printf("---- ans\n");
    cout << ans << endl;
}

///////////////////////////////////////////////////////////////////////////////


Measured memory_bytes: 11056