class Stack:
    def __init__(self):
        self.items = []

    def push(self, v):
        self.items.append(v)

    def pop(self):
        if self.items:
            return self.items.pop()
        return None

    def size(self):
        return len(self.items)


s = Stack()
s.push("a")
s.push("b")
print(s.size())
print(s.pop())
print(s.pop())
print(s.pop())
print(s.size())