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

#define int ll
#define fast_io cin.tie(0)->sync_with_stdio(0);
#define endl '\n'
typedef long long ll;

int32_t main() {
	fast_io;

	int n; cin >> n;
	vector<pair<int, int>> a(n);
	for (auto &[x, y] : a) cin >> x >> y;

	vector<int> ord(n);
	iota(ord.begin(), ord.end(), 0);
	sort(ord.begin(), ord.end(), [&] (int i, int j) { return a[i] < a[j]; });

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