PetarsGame

using System;
using System.Numerics;
using System.Text;

class PetersGame
{
    static void Main()
    {
        ulong starting = ulong.Parse(Console.ReadLine()); //10
        ulong end = ulong.Parse(Console.ReadLine()); //14
        string replacement = Console.ReadLine(); //a

        BigInteger sum = 0;
        string replays = string.Empty;
        string result = string.Empty;

        for (ulong i = starting; i < end; i++)
        {
            if (i % 5 == 0)
            {
                sum += i;
            }
            else
            {
                sum += i % 5;
            }
        }

        replays = sum.ToString();
        if (sum % 2 == 0)
        {
            char toReplace = replays[0];
            result = ReplaceInString(toReplace, replays, replacement);
        }
        else
        {
            char toReplace = replays[replays.Length - 1];
            result = ReplaceInString(toReplace, replays, replacement);
        }
        Console.WriteLine(result);
    }

    public static string ReplaceInString(char toReplace, string replays, string inputReplace)
    {
        string result = "";
        for (int i = 0; i < replays.Length; i++)
        {
            if (replays[i] == toReplace)
            {
                result += inputReplace;
            }
            else
            {
                result += replays[i];
            }
        }

        return result;
    }
}