#include<bits/stdc++.h>
#define ll long long
#define rll register ll
#define ull unsigned ll
#define ri register int
#define fo(i,x,y) for(ri i=(x);i<=(y);++i)
#define fu(i,x,y) for(ri i=(x);i<(y);++i)
#define fd(i,x,y) for(ri i=(x);i>=(y);--i)
#define pb push_back
#define pii pair<int,int>
#define fi first
#define se second
using namespace std;
const int N=10015,M=5005;
int n,l[M],r[M],S,T,cur[N],dis[N],Q[N],ans;
bitset<N>G[N];
inline void link(ri x,ri y){
	G[x].set(y);G[y].set(x);
}
inline int bfs(){
	ri h=0,t=1;Q[1]=S;
	fo(i,1,T)dis[i]=1e9;
	dis[S]=0;
	while(h<t){
		ri x=Q[++h],y=G[x]._Find_first();
		while(y<=T){
			if(!G[x][y])break;
			if(dis[y]==1e9){
				dis[y]=dis[x]+1;
				Q[++t]=y;
			}
			ri z=G[x]._Find_next(y);
			y=max(y+1,z);
		}
	}
	return dis[T]!=1e9;
}
inline int dfs(ri x,ri fw){
	if(x==T||!fw)return fw;
	ri res=0,y=G[x]._Find_first();
	while(y<=T&&fw){
		if(!G[x][y])break;
		if(dis[y]==dis[x]+1){
			ri Fw=dfs(y,1);
			if(!Fw)dis[y]=1e9;
			else{
				G[x].flip(y);
				G[y].flip(x);
				res+=Fw;fw-=Fw;
			}
		}
		ri z=G[x]._Find_next(y);
		y=max(y+1,z);
	}
	cur[x]=y;
	return res;
}
int main(){
	scanf("%d",&n);
	S=2*n+1;T=S+1;
	fo(i,1,n){
		scanf("%d%d",&l[i],&r[i]);
		link(S,l[i]);link(r[i],T);
	}
	fo(i,1,n)fo(j,1,n){
		if(l[i]<l[j]&&r[i]<r[j]&&l[j]<r[i])link(l[j],r[i]);
	}
	while(bfs()){
		ri fw=0;
		fo(i,1,T)cur[i]=G[i]._Find_first();
		while(fw=dfs(S,1e9))ans+=fw;
	}
	printf("%d",2*n-ans);
}