Petar's game

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


namespace novo2
{
    class Program
    {
        static void Main(string[] args)
        {
            ulong startNumber = ulong.Parse(Console.ReadLine());
            ulong endNumber = ulong.Parse(Console.ReadLine());
            string givenString = Console.ReadLine();
            BigInteger sum = 0;
            for (ulong i = startNumber; i < endNumber; i++)
            {
                if (i % 5 == 0)
                {
                    sum += i;
                }
                else
                {
                    sum += i % 5;
                }
            }
            if (sum % 2 == 0)
            {
                string sumString = sum.ToString();
                string element = string.Empty;
                element += sumString[0];
                sumString = sumString.Replace(element, givenString);
                Console.WriteLine(sumString);

                }
            else
            {
                string sumString = sum.ToString();
                string element = string.Empty;
                element += sumString[sumString.Length - 1];
                sumString = sumString.Replace(element, givenString);
                Console.WriteLine(sumString);
            }
            }
        }
    }