Print Long Sequence

using System;

namespace Problem_16__PrintLongSequence
{
    class PrintLongSequence
    {
        public static void Main ()
        {
            Console.WriteLine ("Please enter the length of the sequence:");
            int len;
            string input = Console.ReadLine ();
            bool checkInput = int.TryParse (input, out len);

            if (checkInput && len > 0)
            {
                string output = "";

                for (int i = 2; i <= len; i++) {
                    int num = (i % 2 == 0) ? i : (i * (-1));
                    output += (i != (len)) ? num + ", " : num.ToString ();
                }
                Console.WriteLine ("{0}", output);
            }
            else
            {
                Console.WriteLine ("Please enter a valid length of the sequence! It should be a 32 bit positive Intereger!");
            }
        }
    }
}