import java.util.ArrayList;
import java.util.Arrays;
import java.util.Scanner;

class Stack {

    private int pointer;
    private int[] stack;
    private int size;


    public Stack() {
        this(20);
    }

    public Stack(int capacity) {
            size = capacity;
            stack = new int[size];
            pointer = -1;
    }

    public void push(int element) {
        stack[++pointer] = element;
    }

    public int pop() {
        int i = stack[pointer];
        stack[pointer--] = 0;
        return i;
    }

    public int back() {
        return stack[pointer];
    }

    public void clear() {
        Arrays.fill(stack, 0);
        pointer = -1;
    }

    public boolean isEmpty() {
        if (pointer < 0)
            return true;
        else
            return false;
    }

    public int size() {
        return this.pointer + 1;
    }

    public static void main(String[] args) {
        Stack stack = new Stack();

        Scanner scanner = new Scanner(System.in);
        System.out.println("enter your command");
        String command = scanner.nextLine();

        while (!command.equals("exit")) {
            if (command.matches("^push [0-9]")) {
                String[] comm = command.split(" ");
                stack.push(Integer.parseInt(comm[1]));
                System.out.println("OK");
            } else if (command.equals("pop")) {
                if (stack.isEmpty()) {
                    System.out.println("Error. Stack is empty");
                } else {
                    System.out.println(stack.pop());
                }
            } else if (command.equals("size")) {
                System.out.println(stack.size());
            } else if (command.equals("back")) {
                if (stack.isEmpty()) {
                    System.out.println("Error. Stack is empty");
                } else {
                    System.out.println(stack.back());
                }
            } else if (command.equals("clear")) {
                stack.clear();
                System.out.println("OK");
            } else {
                System.out.println("Command doesn't exist");
            }
            command = scanner.nextLine();
        }
        System.out.println("bye");

    }
}