Check sum algorithm implementation

CheckSum.cs

using System;
using System.Linq;

namespace CSLanguage
{
    class Program
    {
        static int DigitsSum(string digits)
        {
            int tmp = 0;
            foreach (char c in digits)
                tmp += c - '0';
            return tmp;
        }

        static int CheckSum(int number)
        {
            var digits = number.ToString().ToArray();

            for (var i = digits.Length - 1; i >= 0; i -= 2)
            {
                var str = ((digits[i] - '0') * 2).ToString();
                digits[i] = (char)(DigitsSum(str) + '0');
            }
            var result = 10 - DigitsSum(new string(digits)) % 10;
            return result;
        }

        static int LinqCheckSum(int number)
        {
            var result=
            number.ToString()
                .Reverse()
                .Select(x => x - '0')
                .Zip(Enumerable.Range(0, 9), (x, y) => new {x, y})
                .Select(x => x.y%2 == 0 ? x.x*2 : x.x)
                .Select(x => x.ToString().Sum(y => y - '0'))
                .Reverse()
                .Sum();

            return 10 - result%10;

        }

        static void Main(string[] args)
        {
            int x = int.Parse(Console.ReadLine());

            Console.WriteLine(CheckSum(x));

            Console.WriteLine(LinqCheckSum(x));

        }
    }
}