import sys

def push(stack, number):
    stack.append(number)
    print('ok')

def pop(stack):
    if not stack:
        print('error')
    else:
        last_element = stack[-1]
        del stack[-1]
        print(last_element)

def back(stack):
    if not stack:
        print('error')
    else:
        print(stack[-1])

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

def clear(stack):
    del stack[:]
    print('ok')

stack = []
while True:
    data = input()
    if 'push' in data:
        n = data.split()[1]
        push(stack, n)
    elif data == 'pop':
        pop(stack)
    elif data == 'back':
        back(stack)
    elif data == 'size':
        size(stack)
    elif data == 'clear':
        clear(stack)
    elif data == 'exit':
        print('bye')
        sys.exit()