Petar's Game

using System;
using System.Numerics;


class Program
{
    static void Main(string[] args)
    {
        ulong startNum = ulong.Parse(Console.ReadLine());
        ulong endNum = ulong.Parse(Console.ReadLine());
        string str = Console.ReadLine();
        BigInteger sum = 0L;
        for (ulong i = startNum; i < endNum; i++)
        {
            if (i % 5 == 0)
            {
                sum += i;
            }
            else
            {
                sum += i % 5;
            }
        }
        string sumAsString = sum + "";
        string result = "";


        if (sum % 2 == 0)
        {
            string testlet = sumAsString[0].ToString();
            for (int i = 0; i < sumAsString.Length; i++)
            {
                if (testlet != (sumAsString[i].ToString()))
                {
                    result += sumAsString[i];
                }
                else
                {
                    result += str;
                }
            }
            Console.WriteLine(result);

            return;
        }
        else
        {
            string testlet = sumAsString[sumAsString.Length - 1].ToString();
            for (int i = 0; i < sumAsString.Length; i++)
            {
                if (testlet != (sumAsString[i].ToString()))
                {
                    result += (sumAsString[i] + "");
                }
                else
                {
                    result += str;
                }
            }
            Console.WriteLine(result);
        }
    }
}