Simple Stack

Design and implement the data structure stack. Write a program that describes the stack and simulates stack operations, implementing all the methods specified below. The program reads a sequence of commands and, depending on the command, performs the corresponding operation. After executing each command, the program must print one line of output. The possible commands are:

• push $n$ — Add the number $n$ to the stack (the value $n$ is given after the command). Print ok.

• pop — Remove the last element from the stack. Print its value.

• back — Print the value of the last element without removing it from the stack.

• size — Print the number of elements in the stack.

• clear — Clear the stack and print ok.

• exit — Print bye and terminate the program.

It is guaranteed that the sequence of input commands satisfies the following requirements: the maximum number of elements in the stack at any time does not exceed $100$, and all pop and back commands are correct, that is, at the moment they are performed, the stack contains at least one element.