"""
  class Node:
    def __init__(self, x):
      self.left = None
      self.right = None
      self.val = x
"""

def binary_tree_height(node):
  if not node:
    return 0

  left_subtree_height = binary_tree_height(node.left)
  right_subtree_height = binary_tree_height(node.right)

  return max(left_subtree_height, right_subtree_height) + 1