private static void Main(string[] args)
        {
            Console.WriteLine("Enter credit card number without spaces:");
            String CN = Console.ReadLine();
            int[] number = new int[CN.Length];
            for (int i = 0; i < CN.Length; i++)
            {
                number[i] = Convert.ToInt32(CN[i].ToString(), 10);
            }
            int sum = 0;
            for (int i = 0; i < 16; i += 2)
            {
                number[i] = number[i] * 2;
                if (number[i] >= 10)
                    sum += 1 + number[i] % 10;
                else
                    sum += number[i];
                sum += number[i + 1];
            }
            if (sum%10==0) Console.WriteLine("Code is valid");
            else Console.WriteLine("Code is invalid");
            Console.ReadKey();
        }