class DoublyLinkedBase:
    class Node:
        __slots__ = '_element', '_prev', '_next'

        def __init__(self, element, prev, next):
            self._element = element
            self._prev = prev
            self._next = next

    def __init__(self):
        self._header = self.Node(None, None, None)
        self._trailer = self.Node(None, None, None)
        self._header._next = self._trailer
        self._trailer._prev = self._header
        self._size = 0

    def __len__(self):
        return self._size

    def is_empty(self):
        return self._size == 0

    def _insert_between(self, e, predecessor, successor):
        newest = self.Node(e, predecessor, successor)
        predecessor._next = newest
        successor._prev = newest
        self._size += 1

    def insert_at_position(self, pos, e):
        if pos < 1 or pos > self._size + 1:
            print("Invalid position")
            return

        current = self._header
        for _ in range(1, pos):
            current = current._next

        self._insert_between(e, current, current._next)
        print("The element", e, "is successfully inserted at position", pos)

    def _delete_node(self, node):
        predecessor = node._prev
        successor = node._next
        predecessor._next = successor
        successor._prev = predecessor
        self._size -= 1
        element = node._element
        node._prev = node._next = node._element = None
        return element

    def delete_by_position(self, pos):
        if pos < 1 or pos > self._size:
            print("Invalid position")
            return

        current = self._header._next
        for _ in range(1, pos):
            current = current._next

        removed = self._delete_node(current)
        print("The element", removed, "is successfully removed")

    def display(self):
        if self.is_empty():
            print("Doubly linked list is Empty!")
        else:
            current = self._header._next
            print("Elements in Doubly linked list:")
            while current != self._trailer:
                print(current._element, "<->", end=" ")
                current = current._next
            print("NULL")


print("\t\tImplementation of Doubly Linked List ADT")
print("\t\t---------------------------------------")

dll = DoublyLinkedBase()

while True:
    print("\n\tOPERATIONS")
    print("1-Insertion at position")
    print("2-Delete at position")
    print("3-Length")
    print("4-Is Empty")
    print("5-Display")
    print("6-Exit")

    try:
        ch = int(input("Enter your choice: "))

        if ch == 1:
            value = int(input("Enter an element: "))
            pos = int(input("Enter position: "))
            dll.insert_at_position(pos, value)

        elif ch == 2:
            if dll.is_empty():
                print("Doubly linked list is empty!")
            else:
                pos = int(input("Enter position to delete: "))
                dll.delete_by_position(pos)

        elif ch == 3:
            print("Number of elements:", len(dll))

        elif ch == 4:
            print("Is list empty?", dll.is_empty())

        elif ch == 5:
            dll.display()

        elif ch == 6:
            print("Program Exited")
            break

        else:
            print("Invalid choice")

    except ValueError:
        print("Please enter a valid number")