class StackMachine:
	def __init__(self):
		self._data = []

	def push(self, value):
		self._data.append(value)

	def pop(self):
		if not _data:
			return None
		else:
			return _data.pop()

	def add(self):
		try:
			val = _data.pop() + _data.pop()
			return val
		except IndexError:
			return

	def sub(self):
		try:
			val = _data.pop() - _data.pop()
			return val
		except IndexError:
			return

	def mul(self):
		try:
			val = _data.pop() * _data.pop()
			return val
		except IndexError:
			return


	def div(self):
		try:
			val = _data.pop() / _data.pop()
			return val
		except IndexError:
			return


	def mod(self):
		try:
			val = _data.pop() % _data.pop()
			return val
		except IndexError:
			return