#include<bits/stdc++.h>
using namespace std;
int m,n,xx,yy;
bool book[105][105];
bool used[105][105];
int kx[5]={0,0,1,-1};
int ky[5]={1,-1,0,0};
struct oppo{
    int x,y;
}belong[105][105];
int ans;
bool find(int x,int y)
{
    for(int i=0;i<4;i++)
    {
        int xx=x+kx[i];
        int yy=y+ky[i];
        if(xx>0&&yy>0&&xx<=m&&yy<=m&&!book[xx][yy]&&!used[xx][yy])
        {
            used[xx][yy]=1;
            if(belong[xx][yy].x==0||find(belong[xx][yy].x,belong[xx][yy].y))
            {
                belong[xx][yy].x=x;
                belong[xx][yy].y=y;
                return 1;
            }
        }
    }
    return 0;
}
int main()
{
    cin>>m>>n;
    while(n--){
        cin>>xx>>yy;
        book[xx][yy]=1;
    }
    for(int i=1;i<=m;i++)
        for(int j=1;j<=m;j++)
            if((i+j)%2&&!book[i][j])
            {
                memset(used,0,sizeof(used));
                if(find(i,j))
                    ans++;
            }
    cout<<ans<<endl;
    return 0;
}