SimpleLoop

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

namespace ConsoleApplication2
{
    class Program
    {
        static void Main(string[] args)
        {
            BigInteger result = new BigInteger();

            int memberOne = int.Parse(Console.ReadLine());
            int memberTwo = int.Parse(Console.ReadLine());
            int memberThree = int.Parse(Console.ReadLine());
            int memberAdd = int.Parse(Console.ReadLine());

            BigInteger first = memberOne;
            BigInteger second = memberTwo;
            BigInteger third = memberThree;

            for (int i = 4; i <= memberAdd; i++)
            {
                result = first + second + third;
                first = second;
                second = third;
                third = result;
            }

            Console.WriteLine(result);
        }
    }
}