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

template<class T> istream& operator >> (istream& is, vector<T>& vec) {
    for(T& x : vec) is >> x;
    return is;
}

template<class T> ostream& operator << (ostream& os, const vector<T>& vec) {
    if(vec.empty()) return os;
    os << vec[0];
    for(auto it = vec.begin(); ++it != vec.end(); ) os << ' ' << *it;
    return os;
}

int main(){
    ios::sync_with_stdio(false);
    cin.tie(0);
    int n, x, y, id;
    cin >> n;
    vector<tuple<int,int,int>> a(n);
    for(int i = 0; i < n; i++){
        cin >> x >> y;
        a[i] = make_tuple(x, y, i);
    }

    sort(a.begin(), a.end());
    cout << n / 2 << '\n';
    for(int i = 0; i + 1 < n; i += 2){
        int x, y, id;
        int x2, y2, id2;
        tie(x, y, id) = a[i];
        tie(x, y, id2) = a[i + 1];
        cout << id + 1 << ' ' << id2 + 1 << '\n';
    }
}