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

using dog = tuple<int, int, int>;

int main() {
    int n;
    cin >> n;

    vector<dog> dogs;
    for(int i = 1, x, y; i <= n; i ++) {
        cin >> x >> y;
        dogs.push_back({x, y, i});
    }
    sort(dogs.begin(), dogs.end());
    cout << n / 2 << "\n";
    for(int i = 1; i < n; i += 2) {
        cout << get<2>(dogs[i - 1]) << " " << get<2>(dogs[i]) << "\n";
    }
}