#include <iostream>
#include <vector>
#include <cmath>
#include <map>
#include <set>
#include <iomanip>
#include <queue>
#include <algorithm>
#include <numeric>
#include <deque>
#include <complex>
#include <cassert>

using namespace std;
using ll = long long;

int main(){

    int N, x, y, a, b;
    cin >> N;
    vector<tuple<int, int, int>> xy(N);

    for (int i=0; i<N; i++){
        cin >> x >> y;
        xy[i] = {x, y, i+1};
    }

    sort(xy.begin(), xy.end());
    cout << N/2 << endl;
    for (int i=0; i<N/2; i++){
        tie(x, y, a) = xy[i*2];
        tie(x, y, b) = xy[i*2+1];
        cout << a << " " << b << endl;
    }

    return 0;
}