code: def __get_depth(root):
      """
      return 0 if unbalanced else depth + 1
      """
      if root is None:
          return 0
      left = __get_depth(root.left)
      right = __get_depth(root.right)
      if abs(left - right) > 1 or -1 in [left, right]:
          return -1
      return 1 + max(left, right)
  
label_desc: Use image node_id 3 for calling a global function with example usage: __get_depth(root) and returns: unknown, int, unknown