#include<cstdio>

 #include<algorithm>

 #include<cstring>

 #include<iostream>

 #include<cmath>

 #include<cctype>

 #include<set>

 #include<queue>

 #include<vector>

 #include<map>

 using namespace std;

 typedef long long LL;

 inline int read() {

     int x=,f=;char ch=getchar();for(;!isdigit(ch);ch=getchar())if(ch=='-')f=-;

     for(;isdigit(ch);ch=getchar())x=x*+ch-'';return x*f;

 }

 const int N = ;

 LL s1[N], s2[N], a[N], L, Ans;

 int n;

 LL f(int l,int r) {

     return s1[r] - a[l] * (r - l) - s1[l];

 }

 void Calc() {

     for (int i = ; i <= n; ++i)

         s1[i] = s1[i - ] + a[i], s2[i] = s2[i - ] + (L - a[n - i + ]);

     for (int i = ; i < n; ++i) {

         LL now = a[i] * (n - i) + a[i];

         int cnt = (n - i) / ;

         if ((n - i) %  == )

             now += s2[cnt] *  + f(i, i + cnt) *  - (a[i + cnt] - a[i]);

         else

             now += s2[cnt + ] *  + f(i, i + cnt) *  - (L - a[n - cnt]);

         Ans = max(Ans, now);

     }

 }

 int main() {

     L = read(); n = read();

     for (int i = ; i <= n; ++i) a[i] = read();

     Calc();

     for (int i = ; i <= n; ++i) a[i] = L - a[i];

     reverse(a + , a + n + );

     Calc();

     cout << Ans;

     return ;

 }