#include<bits/stdc++.h>
using namespace std;

int main(){
	int N;cin>>N;
	vector<int>X(N),Y(N);
	for(int i=0;i<N;++i)cin>>X[i]>>Y[i];
	vector<int>id(N);
	iota(id.begin(),id.end(),0);
	sort(id.begin(),id.end(),[&](int l,int r){
			if(X[l]!=X[r])return X[l]<X[r];
			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<<endl;
}