0...100 in words

using System;
using System.Collections.Generic;

namespace equals
{
    class MainClass
    {
        public static void Main (string[] args)
        {
            var under13 = new Dictionary<int,string> {
                { 0,"zero" },{ 1,"one" },{ 2,"two" },{ 3,"three" },
                { 4,"four" },{ 5,"five" },{ 6,"six" },{ 7,"seven" },
                { 8,"eight" },{ 9,"nine" },{ 10,"ten" },    { 11,"eleven" },
                { 12,"twelve" }
            };
            var under10 = new Dictionary<int,string> {
                { 0,"" },{ 1," one" },{ 2," two" },{ 3," three" },
                { 4," four" },{ 5," five" },{ 6," six" },{ 7," seven" },
                { 8," eight" },{ 9," nine" }
            };
            var above13 = new Dictionary<int,string> {
                { 2,"twen" },{ 3,"thir" },{ 4,"four" },
                { 5,"fif" },{ 6,"six" },{ 7,"seven" },
                { 8,"eigh" },{ 9,"nine" }
            };
            var a = int.Parse (Console.ReadLine ());
            if ((a>(-1))&&(a<101)){
                if(a < 13)
                    Console.WriteLine (under13 [a]);
                else if (a < 20)
                    Console.WriteLine (above13[(a%10)]+"teen");
                else if (a < 100)
                    Console.WriteLine (above13[(a/10)]+"ty"+under10[(a % 10)]);
                else if (a==100)
                    Console.WriteLine ("one hundred");
            }
                else
                    Console.WriteLine ("invalid number");
        }
    }
}