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

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

  left_subtree_nodes = number_of_nodes(node.left)
  right_subtree_nodes = number_of_nodes(node.right)

  return left_subtree_nodes + right_subtree_nodes + 1