// includes {{{
#include<iostream>
#include<iomanip>
#include<algorithm>
#include<vector>
#include<stack>
#include<queue>
#include<map>
#include<set>
#include<tuple>
#include<cmath>
#include<random>
#include<cassert>
// #include<deque>
// #include<multiset>
// #include<bitset>
#include<cstring>
// #include<bits/stdc++.h>
// }}}
using namespace std;
using ll = long long;


// #undef DEBUG
// #define DEBUG
// DEBUG {{{
#include <array>
#include <deque>
#include <iostream>
#include <list>
#include <queue>
#include <stack>
#include <tuple>
#include <valarray>
#include <vector>
// clang-format off
template<int n, class...T> typename enable_if<(n>=sizeof...(T))>::type _ot(ostream &, tuple<T...> const &){}
template<int n, class...T> typename enable_if<(n< sizeof...(T))>::type _ot(ostream & os, tuple<T...> const & t){ os << (n==0?"":", ") << get<n>(t); _ot<n+1>(os, t); }
template<class...T> ostream & operator<<(ostream &o, tuple<T...> const &t){ o << "("; _ot<0>(o, t); o << ")"; return o; }
template<class T, class U> ostream & operator<<(ostream &o, pair<T, U> const &p) { o << "(" << p.first << ", " << p.second << ")"; return o; }
template < class T > ostream &operator<<(ostream &o, const stack<T> &a) { o << "{"; for(auto tmp = a; tmp.size(); tmp.pop()) o << (a.size() == tmp.size() ? "" : ", ") << tmp.top(); o << "}"; return o; }
template <class T, class Container, class Compare > ostream &operator<<(ostream &os, priority_queue<T, Container, Compare> a) { os << "{ (top) "; while(a.size()) os << a.top() << (a.size() == 1 ? "" : ", "), a.pop(); os << " }"; return os; }
template <class T, class Container > ostream &operator<<(ostream &os, queue<T, Container> a) { os << "{ "; while(a.size()) os << a.front() << (a.size() == 1 ? "" : ", "), a.pop(); os << " }"; return os; }
#ifdef DEBUG
#if !defined(DEBUG_OUT)
#define DEBUG_OUT cerr
#endif
#if !defined(DEBUG_LEFT)
#define DEBUG_LEFT "\e[1;36m"
#endif
#if !defined(DEBUG_RIGHT)
#define DEBUG_RIGHT ":\e[m"
#endif
#define dump(...) [&](){auto __debug_tap=make_tuple(__VA_ARGS__);DEBUG_OUT<<DEBUG_LEFT<<__LINE__ << DEBUG_RIGHT << " " <<#__VA_ARGS__<<" = "<<__debug_tap<<endl;}()
template < class T > inline void dump2D(T &d, size_t sizey, size_t sizex) { for(size_t i = 0; i < sizey; i++) { DEBUG_OUT << "\t"; for(size_t j = 0; j < sizex; j++) DEBUG_OUT << d[i][j] << (j + 1 == sizex ? "" : "\t"); DEBUG_OUT << endl; } }
template < class T > inline void dump1D(T &d, size_t sizey) { for(size_t i = 0; i < sizey; i++) { DEBUG_OUT << d[i] << (i + 1 == sizey ? "" : " "); } DEBUG_OUT << endl; }
template < class T, class = typename iterator_traits< decltype(begin(T())) >::value_type, class = typename enable_if<!is_same<T, string>::value>::type > ostream &operator<<(ostream &o, const T &a) { o << "{"; for(auto ite = begin(a); ite != end(a); ++ite) o << (ite == begin(a) ? "" : ", ") << *ite; o << "}"; return o; }
#else
#define dump(...) (42)
#define dump2D(...) (42)
#define dump1D(...) (42)
template < class T, class = typename iterator_traits< decltype(begin(T())) >::value_type, class = typename enable_if<!is_same<T, string>::value>::type > ostream &operator<<(ostream &o, const T &a) { for(auto ite = begin(a); ite != end(a); ++ite) o << (ite == begin(a) ? "" : " ") << *ite; return o; }
#endif
// clang-format on
// }}}



int n;
ll l;
const int N = 2e5 + 10;
ll x[N];
ll revx[N];
ll xsum[N];
ll revxsum[N];

ll rangeX(int le, int ri) {
  ri = min(ri, n-1);
  if(le > ri) return 0;
  ll res = xsum[ri];
  if(le - 1 >= 0) res -= xsum[le - 1];
  return res;
}

ll rangeXRev(int le, int ri) {
  ri = min(ri, n-1);
  if(le > ri) return 0;
  ll res = revxsum[ri];
  if(le - 1 >= 0) res -= revxsum[le - 1];
  return res;
}

int goal[N];
int revgoal[N];

ll val[N];
ll revval[N];

template<class A, class B, class C, class D, class E>
void calc(A &ans, B &rangeX, C &rangeXRev, D &x, E &revx) {
  for(int i = 0; i < n; i++) {
    ll res = x[i] * 2;
    int f = n - 1 - i;
    int k = f / 2;
    int l = f - k;
    res += rangeX(i + 1, i + 1 + k - 1) * 2;
    res += rangeXRev(0, l - 1) * 2;
    if(f & 1) {
      res -= revx[l - 1];
    } else {
      res -= x[i + 1 + k - 1];
    }
    dump(i, res);
    ans = max(ans, res);
  }
}

int main() {
  ios::sync_with_stdio(false), cin.tie(0);
  cin >> l >> n;
  for(int i = 0; i < n; i++) cin >> x[i], revx[i] = l - x[i];
  reverse(revx, revx + n);

  for(int i = 0; i < n; i++) xsum[i] = x[i];
  for(int i = 0; i < n; i++) revxsum[i] = revx[i];

  for(int i = 1; i < n; i++) xsum[i] += xsum[i-1];
  for(int i = 1; i < n; i++) revxsum[i] += revxsum[i-1];

  ll ans = 0;

  calc(ans, rangeX, rangeXRev, x, revx);
  calc(ans, rangeXRev, rangeX, revx, x);

  cout << ans << endl;
  return 0;
}


Measured memory_bytes: 7168