1 #include<iostream>
 2 #include<cstring>
 3 #include<algorithm>
 4 #define x first
 5 #define y second
 6 using namespace std;
 7 typedef pair<int,int> PII;
 8 const int N = 110;
 9 PII match[N][N];
10 bool st[N][N],bad[N][N];
11 int n,m;
12 int dx[4] = {0,1,0,-1},dy[4] = {1,0,-1,0};
13 bool find(int x,int y){
14     for(int i = 0;i < 4;i ++ ){
15         int a = x + dx[i],b = y + dy[i];
16         if(a < 1 || a > n || b < 1 || b > n) continue;
17         if(st[a][b] || bad[a][b]) continue;
18         PII t = match[a][b];
19         st[a][b] = true;
20         if(t.x == 0 || find(t.x,t.y)){
21             match[a][b] = {x,y};
22             return true;
23         }
24     }
25     return false;
26 }
27 int main(){
28     ios::sync_with_stdio(false);
29     cin.tie(0),cout.tie(0);
30     cin>>n>>m;
31     memset(bad,false,sizeof bad);
32     while(m -- ){
33         int a,b;
34         cin>>a>>b;
35         bad[a][b] = true;
36     }
37     int ans = 0;
38     for(int i = 1;i <= n;i ++ )
39     for(int j = 1;j <= n;j ++ ){
40         if((i + j) % 2 && !bad[i][j]){
41             memset(st,0,sizeof st);
42             if(find(i,j)) ans ++;
43         }
44     }
45     cout<<ans;
46 }