https://www.luogu.com.cn/problem/solution/P8404

#include <bits/stdc++.h>
using namespace std;

const int maxn=1e2+7,inf=0x3f3f3f3f;
int n,m,l,r,u,d,ans=-inf;
struct node{
	int x,y;
}p[maxn];

bool dcmp(node x,node y){
	return x.x==y.x?x.y<y.y:x.x<y.x;
}

bool lcmp(node x,node y){
	return x.y==y.y?x.x<y.x:x.y<y.y;
}

int main(){
	scanf("%d%d",&n,&m);
	for(int i=1;i<=m;i++) scanf("%d%d",&p[i].x,&p[i].y);
	p[++m]=(node){0,0},p[++m]=(node){0,n+1},p[++m]=(node){n+1,0},p[++m]=(node){n+1,n+1};
	sort(p+1,p+1+m,dcmp);
	for(int i=1;i<=m;i++){
		l=1,r=n;
		for(int j=i+1;j<=m;j++){
			if(r-l+1<p[j].x-p[i].x-1) break;
			ans=max(ans,p[j].x-p[i].x-1);
			if(l<=p[j].y&&p[j].y<=p[i].y) l=p[j].y+1;
			else if(r>=p[j].y&&p[j].y>p[i].y) r=p[j].y-1;
		}
	}
	sort(p+1,p+1+m,lcmp);
	for(int i=1;i<=m;i++){
		u=1,d=n;
		for(int j=i+1;j<=m;j++){
			if(d-u+1<p[j].y-p[i].y-1) break;
			ans=max(ans,p[j].y-p[i].y-1);
			if(u<=p[j].x&&p[j].x<=p[i].x) u=p[j].x+1;
			else if(d>=p[j].x&&p[j].x>p[i].x) d=p[j].x-1;
		}
	}
	printf("%d",ans);
	return 0;
}