using System;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        var result = TribonacciNumber(20);

        Console.WriteLine(result);

        for (var i = 0; i < map.Count; i++)

            Console.WriteLine(map[i]);

    }

    public static int TribonacciNumber(int n)

    {

        if (n == 0) return 0;

        if (n == 1 || n == 2) return 1;

        return TribonacciNumber(n-3) + TribonacciNumber(n-2) + TribonacciNumber(n-1);

    }

    static Dictionary<int, int> map = new Dictionary<int, int>();

    public static int TribonacciNumber1(int n)

    {

        if (n == 0) return 0;

        if (n == 1 || n == 2) return 1;

        if (map.ContainsKey(n)) return map[n];

        //  else map[n] = ;

        map[n] = TribonacciNumber1(n-3) + TribonacciNumber1(n-2) + TribonacciNumber1(n-1);      

        return map[n];

    }

}