import sys, collections

queue = collections.deque()
while True:
    data = input()
    if 'push' in data:
        queue.append(data.split()[1])
        print('ok')
    elif data == 'pop':
        if not queue:
            print('error')
        else:
            print(queue.popleft())
    elif data == 'front':
        if not queue:
            print('error')
        else:
            print(queue[0])
    elif data == 'size':
        print(len(queue))
    elif data == 'clear':
        queue.clear()
        print('ok')
    elif data == 'exit':
        print('bye')
        sys.exit()