public class Node {

public String data;
public Node next;

public Node(String data)
	{
    	this.data = data;
	}

public void displayNode()
	{
	    System.out.println(data);
	}
}
//LISTA

class Lista {

    Node first = null;

    public String addFirst(String data) {
        Node newFirst = new Node(data);
        newFirst.next = first;
        first = newFirst;
        return first.data;
    }

    public String removeFirst() {
        Node oldFirst = first;
        first = first.next;
        return oldFirst.data;
    }

    public boolean isEmpty() {
        return first == null;
    }
    public String firstElem()
    {
    	return first.data;
    }
    public void displayList()
    {
        Node current = first;
        while (current != null)
        {
            current.displayNode();
            current = current.next;
        }
    }
}

//STOS
class Stos {

    private final Lista lista = new Lista();

    public String push(String data)
    {
        return lista.addFirst(data);
    }

    public String pop() {
        return lista.removeFirst();
    }

    public boolean isEmpty() {
        return lista.isEmpty();
    }

    public String toString() {
        return lista.toString();
    }
    public String peek()
    {
    	return lista.firstElem();
    }
    public void display()
    {
    	lista.displayList();
    }
}