class StackNode(object):
    def __init__(self, value, nxt):
        self.value = value
        self.next = nxt

    def __repr__(self):
        nval = self.next and self.next.value or None
        return f"[{self.value}:{repr(nval)}]"


class Stack(object):
    def __init__(self):
        self.topmost = None

    def push(self, obj):
        """
        Pushes a new value to the top of the stack.
        Añade un nuevo valor en el tope del stack.
        """
        if not self.topmost:
            self.topmost = StackNode(obj, None)
        else:
            self.topmost = StackNode(obj, self.topmost)

    def pop(self):
        """
        Pops the value that is currently on the top of the stack.
        Remueve el valor que está en el tope del stack.
        """
        if self.topmost:
            node = self.topmost
            if self.topmost.next:
                self.topmost = self.topmost.next
            else:
                self.topmost = None
            return node.value

    def top(self):
        """
        Returns the reference to the first item, does not remove.
        Regresa la referencia al primer ítem sin removerlo.
        """
        if self.topmost:
            return self.topmost.value

    def count(self):
        """
        Returns number of items.
        Regresa la cantidad de ítems.
        """
        count = 0
        if self.topmost:
            node = self.topmost
            while node:
                count += 1
                node = node.next
        return count

    def dump(self, mark="----"):
        """
        Debugging function that dumps the contents of the stack.
        Función de depuración que arroja los contenidos del stack.
        """
        if self.topmost:
            node = self.topmost
            print(node.value)
            while node.next:
                node = node.next
                print(mark)
                print(node.value)