using System;
using static System.Console;
using System.Linq;
using System.Collections.Generic;
class Program
{
    static int NN => int.Parse(ReadLine());
    static int[] NList => ReadLine().Split().Select(int.Parse).ToArray();
    static int[][] NArr(long n) => Enumerable.Repeat(0, (int)n).Select(_ => NList).ToArray();
    public static void Main()
    {
        Solve();
    }
    static void Solve()
    {
        var n = NN;
        var map = NArr(n).Select((xy, id) => new int[] { xy[0], xy[1], id + 1 }).ToList();
        map.Sort((l, r) =>
        {
            var d = l[0].CompareTo(r[0]);
            if (d != 0) return d;
            return l[1].CompareTo(r[1]);
        });
        WriteLine(n / 2);
        for (var i = 0; i + 1 < n; i += 2)
        {
            WriteLine($"{map[i][2]} {map[i + 1][2]}");
        }
    }
}