MatrixShuffle

using System;
using System.Linq;
using System.Text.RegularExpressions;

class MatrixShuffle
{
    static void Main()
    {
        var size = int.Parse(Console.ReadLine());
        var text = Console.ReadLine();
        var matrix = new char[size, size];

        var limit = text.Length;
        var index = 0;
        var row = 0;
        var col = 0;
        while (index < limit)
        {
            while (col < size && matrix[row, col] == 0)
            {
                matrix[row, col++] = text[index++];
            }
            col--;
            row++;
            while (row < size && matrix[row, col] == 0)
            {
                matrix[row++, col] = text[index++];
            }
            row--;
            col--;
            while (col >= 0 && matrix[row, col] == 0)
            {
                matrix[row, col--] = text[index++];
            }
            row--;
            col++;
            while (row >= 0 && matrix[row, col] == 0)
            {
                matrix[row--, col] = text[index++];
            }
            row++;
            col++;
        }

    //first we get all whites and then we get all blacks
        var sentence = GetSentence(matrix, true) + GetSentence(matrix, false);
        var color = IsPalindrome(sentence) ? "#4FE000" : "#E0000F";

        Console.WriteLine("<div style='background-color:{0}'>{1}</div>", color, sentence);
    }

    static string GetSentence(char[,] matrix, bool isWhite)
    {
        var output = string.Empty;
        for (var rows = 0; rows < matrix.GetLength(0); rows++)
        {
            for (var cols = 0; cols < matrix.GetLength(1); cols++)
            {
                if (isWhite && cols % 2 == 0)
                {
                    output += matrix[rows, cols];
                }
                else if (!isWhite && cols % 2 == 1)
                {
                    output += matrix[rows, cols];
                }
            }
            isWhite = !isWhite;
        }

        return output;
    }

    static string GetReversedString(string input)
    {
        var letters = input.ToCharArray();
        letters = letters.Reverse().ToArray();

        return new string(letters);
    }

    static bool IsPalindrome(string input)
    {
        input = Regex.Replace(input, @"[^a-zA-Z]", "");
        var reversed = GetReversedString(input);
        reversed = Regex.Replace(reversed, @"[^a-zA-Z]", "");

        return input.ToLower() == reversed.ToLower();
    }
}