#!/usr/bin/env python

# ---------------------------------------

# author : Geng Jie

# email  : gengjie@outlook.com

#

# Create Time: 2016/3/16 23:38

# ----------------------------------------

class Node:

	def __init__(self, value):

		self.value = value

		self.next = None

class Stack:

	def __init__(self):

		self.top = None

	def push(self, value):

		node = Node(value)

		node.next = self.top

		self.top = node

	def pop(self):

		node = self.top

		self.top = node.next

		return node.value

if __name__ == '__main__':

	stack = Stack()

	# for i in range(10):

	# 	stack.push(i)

	#

	# while stack.top:

	# 	print(stack.pop())

	exp = '{a * [x/(x+y)]}'

	for c in exp:

		if c in '{[(':

			stack.push(c)

		elif c in '}])':

			v = stack.top.value

			if c == '}' and v != '{':

				raise Exception('failed')

			if c == ']' and v != '[':

				raise Exception('failed')

			if c == ')' and v != '(':

				raise Exception('failed')

			stack.pop()

	if stack.top is not None:

		raise Exception('failed')

	print('ok')