using System;

using System.IO;

using System.Security.Cryptography;

namespace Rextester{

    public class Program{

        public static void Main(string[] args){

            Console.WriteLine(modulo10("00010"));

                              }

​

private static string modulo10(string str)

{

    int digito=0;

    int flag=2;

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

    {

        int valor = Convert.ToInt32(str[i].ToString()) * flag;

        int soma = 0;

        if (valor.ToString().Length > 1)

        {

            string svalor = valor.ToString();

            for (int j = 0; j < svalor.Length; j++)

            {

                soma += Convert.ToInt32(svalor[j].ToString());

            }

        }

        else

        {

            soma = valor;

        }

        digito += soma;

        flag = (flag == 2) ? 1 : 2;

    }

    digito = digito % 10;

    digito = 10 - digito;

    if (digito == 10)

    {

        digito = 0;

    }

    return digito.ToString();

}

    }

}