using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        List<int> parent = new List<int>{1, 2, 3};

        List<int> child = parent.Where(a=> a == 4).ToList();

        List<int> finale = new List<int>();

        finale.AddRange(parent);

        finale.AddRange(child);

        Console.WriteLine(finale.Count());

        foreach(var n in finale){

        Console.WriteLine(n);

        }

        indertatIndex();

    }

    public static void indertatIndex()

    {

        var res = new List<List<string>>();

        res.Insert(0, new List<string>{"A", "B"});

        res.Insert(1, new List<string>{"X", "Y"});

        foreach(var n in res)

        {

            var flatern = string.Join(", ", n);

            Console.WriteLine(flatern);

            foreach(var m in n)

            {           

                Console.WriteLine(m);

            }

        }

    }

}