class Empty(Exception):

	pass

class Linklist:

	class _Node:

		# Nonpublic class for storing a linked node

		__slots__='_element','_next'

		def __init__(self,ele,ne):

			self._element=ele

			self._next=ne

	def __init__(self):

		self._head=None

		self._size=0

		self._tail=None

	def __len__(self):

		return self._size

	def is_empty(self):

		return self._size==0

	def add_first(self,e):

		self._head=self._Node(e,self._head)

		if self._size==0:

			self._tail=self._head

		self._size+=1

	def add_last(self,e):

		newlastnode=self._Node(e,None)

		if ~self.is_empty():

			self._tail._next=newlastnode

			self._tail=newlastnode

		else:

			self._tail=newlastnode

		self._size+=1

	def remove_first(self):

		if self.is_empty():

			raise Empty("The linked is empty")

		self._head=self._head._next

		self._size-=1