#include<bits/stdc++.h>
#define FILE "read"
#define INF 1000000000
using namespace std;
const int MAXN=(int)5010;
struct node{int y,next,v,w,rel;}e[200000];
int n,len,S,T,TT,ans,Link[MAXN],vis[MAXN],dis[MAXN],q[MAXN],lastnode[MAXN],lastedge[MAXN],flow[MAXN];
pair<int,int> a[MAXN];
namespace INIT{
	char buf[1<<15],*fs,*ft;
	inline char getc(){return (fs==ft&&(ft=(fs=buf)+fread(buf,1,1<<15,stdin),fs==ft))?0:*fs++;}
	inline int read(){
		int x=0,f=1;  char ch=getc();
		while(!isdigit(ch))  {if(ch=='-')  f=-1;  ch=getc();}
		while(isdigit(ch))  {x=x*10+ch-'0';  ch=getc();}
		return x*f;
	}
}using namespace INIT;
void insert(int x,int y,int v,int w){
	e[++len].next=Link[x];Link[x]=len;e[len].y=y;e[len].v=v;e[len].w=-w;e[len].rel=len+1;
	e[++len].next=Link[y];Link[y]=len;e[len].y=x;e[len].v=0;e[len].w=w;e[len].rel=len-1;
}
bool spfa(){
	memset(vis,0,sizeof(vis));
	memset(dis,10,sizeof(dis));
	int head=0,tail=1,oo=dis[0]; 
	q[1]=S; dis[S]=0; vis[S]=1; flow[S]=INF;
	while(head!=tail){
		int x=q[++head]; vis[x]=0;
		if(head==5000) head=0;
		for(int i=Link[x];i;i=e[i].next){
			if(e[i].v&&e[i].w+dis[x]<dis[e[i].y]){
				dis[e[i].y]=e[i].w+dis[x];
				flow[e[i].y]=min(e[i].v,flow[x]);
				if(!vis[e[i].y]){
					vis[e[i].y]=1;
					q[++tail]=e[i].y;
					if(tail==5000) tail=0;
				}
				lastnode[e[i].y]=x;  lastedge[e[i].y]=i;
			}
		}
	}
	if(dis[T]==oo) return 0;
	ans+=flow[T]*(-dis[T]);
	for(int i=T;i!=S;i=lastnode[i]){
		e[lastedge[i]].v-=flow[T];
		e[e[lastedge[i]].rel].v+=flow[T];
	}
	return 1;
}
void solve(){
	while(spfa());
	printf("%d\n",ans);
}
int main(){
	freopen(FILE".in","r",stdin);
	freopen(FILE".out","w",stdout);
	n=read(); S=n*2+1;  TT=S+1;  T=TT+1;
	for(int i=1;i<=n;++i){
		a[i].first=read(); a[i].second=read();
		insert(0,i,2,0);  
		insert(i,i+n,1,1);
		insert(i,i+n,1,0);
		insert(i+n,TT,2,0);
	}
	sort(a+1,a+n+1);
	for(int i=1;i<=n;++i){
		int maxx=INF;
		for(int j=i+1;j<=n;++j)
			if(a[j].second>=a[i].second&&a[j].second<maxx){
				insert(i+n,j,2,0);
				maxx=a[j].second;
			}
	}
	insert(S,0,2,0); insert(TT,T,2,0);
	solve();
	return 0;
}

#include<bits/stdc++.h>
#define FILE "read"
using namespace std;
map<pair<int,int>,int>M;
int main(){
	freopen(FILE".in","w",stdout);
	srand(time(NULL));
	int n=2000; printf("%d\n",n);
	/*for(int i=1;i<=n/2;++i){
		printf("%d %d\n",i,i);
	}*///构造数据
	for(int i=1;i<=n;++i){
		int x=rand()%1000+1,y=rand()%1000+1;
		while(M[make_pair(x,y)]) x=rand()%1000+1,y=rand()%1000+1;
		M[make_pair(x,y)]=1;
		printf("%d %d\n",x,y);
	}
	return 0;
}