using System;

public abstract class Stack{ 
  
  abstract public void Push(Object el);

  abstract public void Pop();

  abstract public Object Top{
    get;}

  abstract public bool Full{
    get;}

  abstract public bool Empty{
    get;}

  abstract public int Size{
    get;}   

  public void ToggleTop(){
    if (Size >= 2){
      Object topEl1 = Top;  Pop();
      Object topEl2 = Top;  Pop();
      Push(topEl1); Push(topEl2);
    }
  }   

  public override String ToString(){
    return String.Format("Stack[{0}]", Size );
  }   
}