#include <bits/stdc++.h>
using namespace std;
using T = tuple<int, int, int>;
int main () {
	int N;
	cin >> N;
	std::vector<T> A(N);
	int s = 0;
	for (auto& [a, b, c] : A) {
		cin >> a >> b;
		c = ++s;
	}
	sort(A.begin(), A.end());
	cout << N / 2 << endl;
	for (int i = 0; i < N / 2; i ++) {
		printf("%d %d\n", get<2>(A[i * 2]), get<2>(A[i * 2 + 1]));
	}
}