Tribonacci.cs

using System;
using System.Numerics;

class Tribonacci
{
    static void Main()
    {
        BigInteger tribo1 = BigInteger.Parse(Console.ReadLine());
        BigInteger tribo2 = BigInteger.Parse(Console.ReadLine());
        BigInteger tribo3 = BigInteger.Parse(Console.ReadLine());
        BigInteger current = 0;
        BigInteger n = BigInteger.Parse(Console.ReadLine());
        if (n == 1)
        {
            Console.WriteLine(tribo1);
        }
        else if (n == 2)
        {
            Console.WriteLine(tribo2);
        }
        else if (n == 3)
        {
            Console.WriteLine(tribo3);
        }
        else
        {
            for (BigInteger i = 0; i < n - 3; i++)
            {
                current = tribo1 + tribo2 + tribo3;
                tribo1 = tribo2;
                tribo2 = tribo3;
                tribo3 = current;
            }
            Console.WriteLine(current);
        }
    }
}