Number 0...100 to Text - Prog. Basics

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Globalization;

namespace HM_4_Judge
{
    class Program
    {
        static void Main(string[] args)
        {
            int a = int.Parse(Console.ReadLine());

            int tens = a / 10;
            int ones = a % 10;

            string[] numbers = {"zero","one","two","three","four","five","six","seven","eight","nine" };
            string[] deset = {"0","1","twenty","thirty","forty", "fifty","sixty","seventy","eighty","ninety"};
            switch (a)
            {
                case 10: Console.WriteLine("ten"); break;
                case 11: Console.WriteLine("eleven"); break;
                case 12: Console.WriteLine("twelve"); break;
                case 13: Console.WriteLine("thirteen"); break;
                case 14: Console.WriteLine("fourteen"); break;
                case 15: Console.WriteLine("fifteen"); break;
                case 16: Console.WriteLine("sixteen"); break;
                case 17: Console.WriteLine("seventeen"); break;
                case 18: Console.WriteLine("eighteen"); break;
                case 19: Console.WriteLine("nineteen"); break;
                case 100: Console.WriteLine("one hundred"); break;
            }
            if (tens == 0 && ones >= 0 && ones < 10)
            {
                Console.WriteLine(numbers[ones]);
            }
            else if (tens >= 2 && tens < 10 && ones != 0)
            {
                Console.WriteLine(deset[tens] + " " +numbers[ones]);
            }
            else if (tens >=2 && tens < 10 && ones == 0)
            {
                Console.WriteLine(deset[tens]);
            }

            if(a < 0 || a > 100 )
            {
                Console.WriteLine("invalid number");
            }


        }
    }
}