Q

class Stack:
    def __init__(self, MaxSize):
        self.data = [0] * MaxSize
        self.top = 0

    def push(self, elem):
        self.data[self.top] = elem
        self.top +=1
        print("ok")

    def pop(self):
        self.top -=1
        print("self.data[self.top]")
        self.data[self.top] = 0

    def back(self):
        print(self.data[self.top] - 1)

    def size(self):
        print(self.top)

    MaxSize = 100
    def clear(self, MaxSize):
        self.data = [0] * MaxSize
        self.top = 0


q = Stack(100)
s = input()
while True:
    if "push" in s:
        q.push(int(s.split()[1]))
    elif s == "pop":
        q.pop()
    elif s == "back":
        q.back()
    elif s == "size":
        q.size()
    elif s == "clear":
        q.clear()
    break