#include <bits/stdc++.h>

using namespace std;
const int maxn = 105;
int n, t;
int dx[4] = {0,0,1,-1}, dy[4] = {1,-1,0,0};
bool valid[maxn][maxn], vis[maxn][maxn];
int match[maxn][maxn]; //match = xxxxxx = x * 1000 + y

bool ok(int x, int y){
    if(x >= 1 && x <= n && y >= 1 && y <= n && valid[x][y] == 0) return true;
    return false;
}

bool dfs(int x, int y){
    for(int i = 0; i < 4; ++ i){
        int xx = x + dx[i], yy = y + dy[i];
        if(ok(xx,yy) && vis[xx][yy] == 0){
            vis[xx][yy] = 1;
            if(match[xx][yy] == 0 || dfs(match[xx][yy] / 1000, match[xx][yy] % 1000)){
                match[xx][yy] = x * 1000 + y;
                return true;
            }
        }
    }
    return false;
}

int main(){
    cin >> n >> t;
    for(int i = 1, x, y; i <= t; ++ i){
        cin >> x >> y;
        valid[x][y] = 1;
    }
    int ans = 0;
    for(int i = 1; i <= n; ++ i)
        for(int j = 1; j <= n; ++ j)
            if((i + j) % 2 == 0 && valid[i][j] == 0){
                memset(vis, 0, sizeof(vis));
                if (dfs(i, j)) ans++;
            }

    cout << ans << endl;
}