0-100.cs

namespace SomeNumbers
{
    using System;

    public static class Program
    {
        private static readonly string[] NumbersInWords =
        {
            "zero", "one", "two", "three", "four", "five", "six", "seven", "eight", "nine", "ten", "eleven", "twelve", "thirteen", "fourteen", "fifteen", "sixteen", "seventeen", "eighteen", "nineteen", "twenty", "thirty", "fourty", "fifty", "sixty", "seventy", "eighty", "ninety"
        };

        public static void Main()
        {
            var num = int.Parse(Console.ReadLine());
            var result = string.Empty;
            if (num < 0 || num > 100)
            {
                result += "invalid number";
            }
            else if (num == 100)
            {
                result += "one hundred";
            }
            else
            {
                var firstNum = (num / 10) % 10;
                var secondNum = num % 10;
                if (num > 20)
                {
                    result += NumbersInWords[(firstNum - 2) + 20];
                    if (secondNum != 0)
                    {
                        result += " " + NumbersInWords[secondNum % 10];
                    }
                }
                else
                {
                    result += NumbersInWords[num];
                }
            }

            Console.WriteLine(result);
        }
    }
}