易错点：

• 八倍开边/加边剪枝(x+y&1)二选一.
• 只有坐标不越界的点集才是好同志.
• 支持相互match这个一定要好评.

#include<cstdio>
#include<iostream>
#include<cstring>
using namespace std;
const int MAXN=120*120;
int dx[8]={-2,-2,-1,-1,1,1,2,2},dy[8]={1,-1,2,-2,2,-2,1,-1};
struct Edge{
    int from,to,nxt;
}e[MAXN*8];
int head[MAXN],edgeCnt=1;
void addEdge(int u,int v){
    e[++edgeCnt].from=u;
    e[edgeCnt].to=v;
    e[edgeCnt].nxt=head[u];
    head[u]=edgeCnt;
}
int vis[MAXN],match[MAXN];
bool dfs(int x){
    for(int i=head[x];i;i=e[i].nxt){
        int nowV=e[i].to;
        if(vis[nowV])continue;
        vis[nowV]=1;
        if(!match[nowV]||dfs(match[nowV])){
            match[nowV]=x;
            match[x]=nowV;
            return 1;
        }
    }
    return 0;
}
int m;
int getHash(int i,int j){
    return (i-1)*m+j;
}
int a[120][120];
int main(){
    int n,t;
    scanf("%d%d%d",&n,&m,&t);
    for(int i=1;i<=t;i++){
        int x,y;
        scanf("%d%d",&x,&y);
        a[x][y]=1;
    }
    for(int x=1;x<=n;x++)
    for(int y=1;y<=m;y++){
        if(a[x][y])continue;
        int nowU=getHash(x,y);
        for(int k=0;k<=7;k++){
            int nowX=x+dx[k],nowY=y+dy[k];
            if(nowX<1||nowX>n||nowY<1||nowY>m)continue;
            if(a[nowX][nowY])continue;
            int nowV=getHash(nowX,nowY);
            addEdge(nowU,nowV);
        }
    }
    int k=0,pointCnt=n*m;
    for(int i=1;i<=pointCnt;i++){
        if(match[i])continue;
        memset(vis,0,sizeof(vis));
        k+=dfs(i);
    }
    int ans=n*m-t-k;
    printf("%d\n",ans);
    return 0;
}