using System;

public class Program

{

    public static void Main()

    {

        var isin = "3535096815902";

        var isinToReverse = isin.ToCharArray();

        Console.WriteLine(isinToReverse);

        Array.Reverse(isinToReverse);

        Console.WriteLine(isinToReverse);

​

        int sum = 0;

​

        for (int i = isinToReverse.Length - 1; i >= 0; i--)

        {

            int.TryParse(isinToReverse[i].ToString(), out var currentNumber);

            int numberToAdd = 0;

            if ((i + 1) % 2 != 0)

            {

                numberToAdd = (currentNumber - '0') * 2;

                Console.WriteLine("Times 2: " + currentNumber + ", gives: " + numberToAdd);

            }

            else

            {

                Console.WriteLine("NumberToAdd: " + numberToAdd);

                Console.WriteLine("CurrentNumber: " + currentNumber);

                numberToAdd = currentNumber;

                Console.WriteLine("NumberToAdd2: " + numberToAdd);

                Console.WriteLine("Times 1: " + currentNumber + ", gives: " + numberToAdd);

            }

​

            if (numberToAdd >= 10)

            {

                Console.WriteLine("Entered for: " + currentNumber + ", giving: " + numberToAdd);

                sum += int.Parse(numberToAdd.ToString().Substring(0, 1));

                sum += int.Parse(numberToAdd.ToString().Substring(1, 1));

            }

            else

            {

                sum += numberToAdd;

            }

        }

​

        var differenceWithHigherTen = (10 - (sum % 10)) % 10;

​

        var result = differenceWithHigherTen == 10 ? 0 : differenceWithHigherTen;

        Console.WriteLine(result);

    }

}