using System;

using System.Linq;

public class Program

{

public static int Mod10Check(string creditCardNumber)

{

    //// 1. Starting with the check digit double the value of every other digit 

    //// 2. If doubling of a number results in a two digits number, add up

    ///   the digits to get a single digit number. This will results in eight single digit numbers                    

    //// 3. Get the sum of the digits

    int sumOfDigits = creditCardNumber.Where((e) => e >= '0' && e <= '9')

                    .Reverse()

                    .Select((e, i) => ((int)e - 48) * (i % 2 == 0 ? 1 : 2))

                    .Sum((e) => e / 10 + e % 10);

​

​

    //// If the final sum is divisible by 10, then the credit card number

    //   is valid. If it is not divisible by 10, the number is invalid.            

    return sumOfDigits % 10;

}

    public static void Main()

    {

​

         Console.WriteLine(Mod10Check("0.5").ToString());

​

    }

}

​

​