using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public static string GetHiddenMessage(char[,] puzzle, string[] keys)

    {

        List<char> charList = puzzle.Cast<char>().ToList();

        foreach(string key in keys)

        {

            for (int i = 0; i < key.Length; i++)

            {

                if (charList.Contains(key[i]))

                {

                    charList.Remove(key[i]);

                }

            }

        }

​

        return new string(charList.ToArray());

    }

    public static void Main()

    {

        Console.WriteLine("UniLecs");

        char[,] puzzle = new char[,]

        {

            { 'U', 'T', 'H', 'E' },

            { 'N', 'A', 'S', 'K' },

            { 'I', 'L', 'E', 'C' },

            { 'L', 'L', 'O', 'S' }

        };

        string[] keys = new string[]

        {

            "TASK", 

            "UNILECS"

        };

        Console.WriteLine(string.Format("Answer = {0}", GetHiddenMessage(puzzle, keys))); // HELLO      

    }

}