class Node:
    def __init__(self,prev,data,nextto):
        self.prev = prev
        self.data = data
        self.next = nextto

class DLL:

    def __init__(self):
        self.head = None
        self.tail = None
        self.curr = None


    def add_to_front(self,value):
        if self.head == None:
            self.head = Node(None,value,self.tail)
        else:
            self.head = Node(None,value,self.head)

    def add_to_back(self,value):
        if self.head == None:
            self.head = Node(None,value,self.tail)
        elif self.tail == None and self.head.next == self.tail:
            self.tail = Node(self.head,value,None)
            self.head = Node(None,self.head.data,self.tail)
        else:

            self.tail.prev = self.tail
            self.tail = Node(self.tail.prev,value,None)
            self.tail.prev.next = self.tail