simple stack

stack = []

def push(x):
    print('ok')
    stack.append(x)

def back():
    print( stack[-1] )

def pop():
    back()
    stack.pop()

def size():
    print( len(stack) )

def clear():
    print('ok')
    stack.clear()

while True:
    command = input()
    if command == 'exit':
        print('bye')
        break
    elif command[:4] == 'push':
        push(command.split()[1])
    else:
        exec( command + "()" )