using System;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        Console.WriteLine(NormalizeVendorCode(@"<<<([:~*@---+=Hell_o_%&^№{/|\};,# $World...?!=+---@*~:])>>>"));

    }

​

    public static string NormalizeVendorCode(string term)

    {

        if (string.IsNullOrWhiteSpace(term))

        {

            throw new ArgumentException("term");

        }

​

        var result = string.Empty;

        var allowedSpecialCharacters = new[]{'+', '#'};

        foreach (var character in term)

        {

            if (char.IsLetterOrDigit(character) || allowedSpecialCharacters.Contains(character))

            {

                result += character;

            }

        }

​

        if (result == string.Empty)

        {

            throw new ArgumentException("term");

        }

​

        return result;

    }

}