[C# Basics Sample Exam May 2014] Tribonacci number

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Numerics;

namespace _02.Simple_Loops
{
    class SimpleLoops
    {
        static void Main(string[] args)
        {
            BigInteger first = BigInteger.Parse(Console.ReadLine());
            BigInteger second = BigInteger.Parse(Console.ReadLine());
            BigInteger third = BigInteger.Parse(Console.ReadLine());
            int numberOfElement = int.Parse(Console.ReadLine());

            if (numberOfElement == 1)
            {
                Console.WriteLine(first);
            }
            else if (numberOfElement == 2)
            {
                Console.WriteLine(second);
            }
            else if (numberOfElement == 3)
            {
                Console.WriteLine(third);
            }
            else
            {
                for (int i = 4; i <= numberOfElement; i++)
                {
                    BigInteger tribonacciNumber = first + second + third;
                    first = second;
                    second = third;
                    third = tribonacciNumber;
                }
                Console.WriteLine(third);
            }
        }
    }
}