#include <bits/stdc++.h>
//#include <atcoder/all>
using namespace std;
//using namespace atcoder;
//using mint = modint1000000007;
//const int mod = 1000000007;
//using mint = modint998244353;
//const int mod = 998244353;
//const int INF = 1e9;
//const long long LINF = 1e18;
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define rep2(i,l,r)for(int i=(l);i<(r);++i)
#define rrep(i, n) for (int i = (n-1); i >= 0; --i)
#define rrep2(i,l,r)for(int i=(r-1);i>=(l);--i)
#define all(x) (x).begin(),(x).end()
#define allR(x) (x).rbegin(),(x).rend()
#define endl "\n"
#define P pair<int,int>
template<typename A, typename B> inline bool chmax(A & a, const B & b) { if (a < b) { a = b; return true; } return false; }
template<typename A, typename B> inline bool chmin(A & a, const B & b) { if (a > b) { a = b; return true; } return false; }
bool comp(const pair<P, int>& lh, const pair<P, int>& rh) {
	if (lh.first.first != rh.first.first)return lh.first.first < rh.first.first;
	return lh.first.second < rh.first.second;
}
int main() {
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	int n; cin >> n;
	vector<pair<P, int>>v(n);
	rep(i, n) {
		cin >> v[i].first.first >> v[i].first.second;
		v[i].second = i;
	}
	sort(all(v), comp);
	cout << v.size() / 2 << endl;
	rep(i, v.size() / 2) {
		cout << v[i * 2].second + 1 << " " << v[i * 2 + 1].second + 1 << endl;
	}
	return 0;
}