using System;

public class Program

{

    public static void Main()

    {

        string documentName = "document1.pdf";

        string extention = "pdf";       

        Console.WriteLine(documentName.Substring(0));

        Console.WriteLine("Substring: "+ documentName.Substring(documentName.Length-extention.Length));

        string name = (string.IsNullOrWhiteSpace(extention) || documentName.Substring(documentName.Length-extention.Length)== extention)? documentName : documentName+"."+extention;

        Console.WriteLine(name);

    }

}