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

typedef modint998244353 mint;
typedef long long ll;

int main(){
	int n; cin >> n;
	vector<tuple<int,int,int>> p;
	for (int i=0; i<n; i++){
		int x, y; cin >> x >> y;
		p.push_back(tuple(x,y,i));
	}
	sort(p.begin(), p.end());

	cout << n/2 << endl;
	for (int i=0; i<n/2; i++){
		cout << get<2>(p[2*i]) + 1 << " " << get<2>(p[2*i+1]) + 1 << endl;
	}
}