using System;

class ClaseA
{
  protected virtual void F()
  {
    Console.WriteLine ("X.F");
  }
  
  protected virtual void F2()
  {
    Console.WriteLine ("X.F2");
  }
}
 
class ClaseB : ClaseA
{
    sealed protected override void F()
    {
        Console.WriteLine ("Y.F");
    }
    
    protected override void F2()
    {
        Console.WriteLine ("X.F3");
    }
}
 
class ClaseC : ClaseB
{
    // Attempting to override F causes compiler error CS0239
    // protected override void F() { Console.WriteLine ("C.F"); }
    
    // Overriding F2 is allowed
    protected override void F2()
    {
        Console.WriteLine ("Z.F2");
    }
    
    public static void Main ()
    {
    	
    }
}