C++ :

#include<iostream>
#include<algorithm>
using namespace std;
const int N = 5005;
struct city{
	int x,y;
};
city a[N];
int f[N];

bool comp(city m,city n){
	return m.x<n.x;
}

int main(){
	int n;
	cin>>n;
	for (int i=1; i<=n; i++) cin>>a[i].x>>a[i].y;
	fill(f,f+N,1);
	sort(a+1,a+n+1,comp);
	for (int i=2; i<=n; i++)
		for (int j=1; j<=i-1; j++)
			if (a[j].y<a[i].y) f[i]=max(f[i],f[j]+1);
	int ans=f[1];
	for (int i=2; i<=n; i++) ans=max(ans,f[i]);
	cout<<ans<<endl;
	return 0;
}