input: Write a Python function to find the middle element of a linked list.
  ```python
  assert find_middle(linked_list_1) == 3
  assert find_middle(linked_list_2) == 20
  assert find_middle(linked_list_3) == 7
output: class ListNode:
      def __init__(self, value=0, next=None):
          self.value = value
          self.next = next
  
  def find_middle(head: ListNode) -> int:
      slow = fast = head
      while fast and fast.next:
          slow = slow.next
          fast = fast.next.next
      return slow.value