Deque

queue = []

while True:
  cmd = input()
  if cmd == "exit":
    print("bye")
    break
  elif cmd == 'pop_front':
    print( queue.pop(0) )
  elif cmd == 'pop_back':
    print( queue.pop() )
  elif cmd == 'front':
    print( queue[0] )
  elif cmd == 'back':
    print( queue[-1] )
  elif cmd == 'size':
    print( len(queue) )
  elif cmd == 'clear':
    queue.clear()
    print('ok')
  else:
    c, n = cmd.split()
    if c == 'push_back':
      queue.append(n)
    else:
      queue.insert(0, n)
    print('ok')