1 /* 1002 */
2 #include <iostream>
3 #include <string>
4 #include <map>
5 #include <queue>
6 #include <set>
7 #include <stack>
8 #include <vector>
9 #include <algorithm>
10 #include <cstdio>
11 #include <cmath>
12 #include <ctime>
13 #include <cstring>
14 #include <climits>
15 #include <cctype>
16 using namespace std;
17
18 const int maxn = 1e3+5;
19
20 int a[maxn], dis[maxn];
21 int n, m;
22
23 int main() {
24 int i, j, k;
25 int mx, q;
26 int l1, l2;
27 int x, y, z, s, tmp;
28 __int64 ans;
29 int L, R;
30
31 #ifndef ONLINE_JUDGE
32 freopen("data.in", "r", stdin);
33 freopen("data.out", "w", stdout);
34 #endif
35
36 while (scanf("%d %d", &n, &m) != EOF) {
37 R = -1;
38 L = n+1;
39 ans = 0;
40 for (i=1; i<=m; ++i) {
41 scanf("%d", &a[i]);
42 L = min(a[i], L);
43 R = max(a[i], R);
44 }
45 if (L==1 || R==n) {
46 s = max(L-1, n-R);
47 for (z=1; z<n; ++z) {
48 if (z >= s-z)
49 break;
50 tmp = s-z;
51 L = (tmp-z)/2+1;
52 R = (tmp+z-1)/2;
53 ans += (R-L+1);
54 }
55 printf("%I64d\n", ans);
56 continue;
57 }
58 l1 = L-1;
59 l2 = n-R;
60 z = l1;
61 for (x=1; x<l2; ++x) {
62 y = l2-x;
63 if (x+y>z && y+z>x && z+x>y)
64 ++ans;
65 }
66 z = l2;
67 for (x=1; x<l1; ++x) {
68 y = l1-x;
69 if (x+y>z && y+z>x && z+x>y)
70 ++ans;
71 }
72 printf("%I64d\n", ans);
73 }
74
75 #ifndef ONLINE_JUDGE
76 printf("%d\n", (int)clock());
77 #endif
78
79 return 0;
80 }
