Error-Correcting Queue

Implement a data structure called a "queue". Write a program that contains the description of the queue and simulates its operation by implementing all methods specified below. The program reads a sequence of commands, performing an operation for each command. After each command, the program should output a single line. The possible commands are:

• push n

  Add the number $n$ to the queue (the value $n$ is specified after the command). The program should output ok.

• pop

  Remove the first element from the queue. The program should output its value.

• front

  Output the value of the first element in the queue without removing it.

• size

  Output the number of elements in the queue.

• clear

  Clear the queue and output ok.

• exit

  Output bye and terminate the program.

Before executing the front or pop operations, the program must check whether the queue contains at least one element. If front or pop is requested and the queue is empty, the program should output the string error instead of a numerical value.