from sys import stdin


class SecureStack(list):
    def push(self, element):
        self.append(element)
        return 'ok'

    def pop(self, __index=-1):
        try:
            return super(SecureStack, self).pop(__index)
        except IndexError:
            return 'error'

    def size(self):
        return len(self)

    def back(self):
        if self.size():
            return self[-1]
        return 'error'

    def exit(self):
        return 'bye'

    def clear(self):
        super(SecureStack, self).clear()
        return 'ok'


s = SecureStack()
for line in stdin:
    command, *arg = line.split()
    print(getattr(s, command)(*arg))
    if command == 'exit':
        break