using System;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        var nip = "5213017228";

        Console.WriteLine("{0}", IsNipValid(nip) ? "valid" : "not walid");

    }

    static bool IsNipValid(string nip)

    {

        var array = nip.Select(c => 

            { 

                return Int32.Parse(c.ToString());

            }).ToList();

​

        if (array.Count != 10)

            return false;

        int[] wagi = new int[] { 6, 5, 7, 2, 3, 4, 5, 6, 7, 0 };

        int suma = array.Zip(wagi, (cyfra, waga) => 

            { 

                return cyfra * waga; 

            })

          .Sum();

​

        return (suma % 11) == (array[9]);

    }

}

​

Program.Main();