#pragma GCC optimize("Ofast",inline)
#include<bits/stdc++.h>
#define ll long long
using namespace std;
int N,vis[5005],link[5005],From[5005],from[5005],L[5005],R[5005],Vis[5005];
bitset<5005> mp[5005],unvis;
queue <int> Q;
int main(){
	scanf("%d",&N);
	for(int i=1;i<=N;++i) cin>>L[i]>>R[i];
	for(int i=1;i<=N;++i) for(int j=1;j<=N;++j) if(L[j]<R[i] && R[i]<R[j] && L[i]<L[j]) mp[i][j]=true;
	int res=0;
	for(int i=1;i<=N;++i) {
		unvis.set();memset(Vis,0,sizeof(Vis)); while(!Q.empty()) Q.pop();
	    Q.push(i);Vis[i]=true;
	    int op=0,now=-1;
	    while(!Q.empty()){
	    	int u=Q.front();Q.pop();
			for(int v=(mp[u]&unvis)._Find_first();v<=N;v=(mp[u]&unvis)._Find_next(v)){
		        unvis[v]=0; From[v]=u;
		        if(!link[v]) { now=v; op=1; break; }
		        if(!Vis[link[v]]) {
		        	Vis[link[v]]=true;
		        	Q.push(link[v]); from[link[v]]=v;
				}
	        }
	        if(op==1) break; 
		}
		res+=op;
		if(op){
			while(1){
				link[now]=From[now];
				if(From[now]==i) break;
				now=from[From[now]];
			}
		}
	}
	printf("%d\n",2*N-res);
	return 0;
}