/* 1002 */

 #include <iostream>

 #include <string>

 #include <map>

 #include <queue>

 #include <set>

 #include <stack>

 #include <vector>

 #include <algorithm>

 #include <cstdio>

 #include <cmath>

 #include <ctime>

 #include <cstring>

 #include <climits>

 #include <cctype>

 using namespace std;

 const int maxn = 1e3+;

 int a[maxn], dis[maxn];

 int n, m;

 int main() {

     int i, j, k;

     int mx, q;

     int l1, l2;

     int x, y, z, s, tmp;

     __int64 ans;

     int L, R;

     #ifndef ONLINE_JUDGE

         freopen("data.in", "r", stdin);

         freopen("data.out", "w", stdout);

     #endif

     while (scanf("%d %d", &n, &m) != EOF) {

         R = -;

         L = n+;

         ans = ;

         for (i=; i<=m; ++i) {

             scanf("%d", &a[i]);

             L = min(a[i], L);

             R = max(a[i], R);

         }

         if (L== || R==n) {

             s = max(L-, n-R);

             for (z=; z<n; ++z) {

                 if (z >= s-z)

                     break;

                 tmp = s-z;

                 L = (tmp-z)/+;

                 R = (tmp+z-)/;

                 ans += (R-L+);

             }

             printf("%I64d\n", ans);

             continue;

         }

         l1 = L-;

         l2 = n-R;

         z = l1;

         for (x=; x<l2; ++x) {

             y = l2-x;

             if (x+y>z && y+z>x && z+x>y)

                 ++ans;

         }

         z = l2;

         for (x=; x<l1; ++x) {

             y = l1-x;

             if (x+y>z && y+z>x && z+x>y)

                 ++ans;

         }

         printf("%I64d\n", ans);

     }

     #ifndef ONLINE_JUDGE

         printf("%d\n", (int)clock());

     #endif

     return ;

 }