这题的转化比较抽象，需要特殊记忆

#include <iostream>
#include <cstring>
#include <algorithm>
using namespace std;
typedef long long LL;
const int N = 110;
typedef pair<int, int> PII;
#define x first
#define y second

PII match[N][N];
bool g[N][N],st[N][N];
int n,m;
int dx[4] = {-1, 0, 1, 0}, dy[4] = {0, 1, 0, -1};

bool find(int x,int y)
{
    for(int i = 0;i<4;i++)
    {
        int a = x+dx[i],b = y+dy[i];
        if(a<1||a>n||b<1||b>n)continue;
        if(st[a][b] || g[a][b])continue;
        st[a][b] = true;
        auto t = match[a][b];
        if(t.x == -1 || find(t.x,t.y))
        {
            match[a][b] = {x,y};
            return true;
        }
    }
    return false;
}
int main()
{
    cin>>n>>m;
    while (m -- )
    {
        int a,b;
        cin>>a>>b;
        g[a][b] = true;
    }

    memset(match,-1,sizeof match);
    int res = 0;
    for(int i = 1;i<=n;i++)
    {
        for(int j = 1;j<=n;j++)
        {
            if((i+j)%2 && !g[i][j])
            {
                memset(st, 0, sizeof st);
                if(find(i,j))res++;
            }
        }
    }
    cout<<res<<endl;
    return 0;
}