flag = 1
while flag == 1:
    s = input().split()
    if s[0] == "push":
        push(int(s[1]))
        print("ok")
    if s[0] == "pop":
        print(pop())
    if s[0] == "back":
        print(back())
    if s[0] == "size":
        print(size())
    if s[0] == "clear":
        clear()
        print("ok")
    if s[0] == "exit":
        flag = 0
        print("bye")