#include<iostream>
#include<algorithm>
using namespace std;
int N,X[8000],Y[8000],id[8000];
int main()
{
	cin>>N;
	for(int i=0;i<N;i++)
	{
		cin>>X[i]>>Y[i];
		id[i]=i;
	}
	sort(id,id+N,[](int l,int r)
	{
		if(X[l]!=X[r])return X[l]<X[r];
		else return Y[l]<Y[r];
	});
	cout<<N/2<<endl;
	for(int i=0;i<N/2;i++)cout<<id[i*2]+1<<" "<<id[i*2+1]+1<<"\n";
}