#include<bits/stdc++.h>
using namespace std;
int main(){
	ios::sync_with_stdio(0);
	cin.tie(0); cout.tie(0);
	int N; cin >> N;
	pair<pair<int, int>, int> A[8010];
	for(int i = 0; i < N; i++){
		cin >> A[i].first.first >> A[i].first.second;
		A[i].second = i+1;
	}
	sort(A, A+N);
	cout << N/2 << "\n";
	for(int i = 0; i < N/2; i++) cout << A[i*2].second << " " << A[i*2+1].second << "\n";
}