class Stack:
	def __init__(self):
		self.__data = []
	
	def empty(self):
		return len(self.__data) == 0
	
	def size(self):
		return len(self.__data)
	
	def push(self, x):
		self.__data.append(x)
	
	def pop(self):
		return self.__data.pop()

stack = Stack()
stack.push(10)
stack.push(15)
stack.push(20)

print(stack.empty())
print(stack.size())
print(stack.pop())
print(stack.pop())
print(stack.pop())
print(stack.empty())
print(stack.size())