import java.io.File;
import java.io.FileNotFoundException;
import java.io.PrintWriter;
import java.util.Scanner;

public class task54 {

    static PrintWriter pw;
    public static void main(String[] args) throws FileNotFoundException{
        Scanner scanner = new Scanner(new File("input.txt"));
        pw = new PrintWriter(new File("output.txt"));
        String command;
        int s;
        MyStack stack = new MyStack(10000);
        do {
            command = scanner.next();
            switch (command) {
            case "push":
                stack.push(scanner.nextInt());
                break;
            case "pop":
                s = stack.pop();
                pw.println((s == -999999999) ? ("error") : (s));
                break;
            case "back":
                pw.println((stack.back() == -999999999) ? ("error") : (stack.back()));
                break;
            case "size":
                pw.println(stack.size());
                break;
            case "clear":
                stack.clear();
                break;
            case "exit":
                stack.exit();
                break;
            default:
                break;
            }
        } while (!command.equals("exit"));
        scanner.close();
        pw.close();
    }

    static class MyStack {
        private int maxSize;
        private int[] stackArray;
        private int top;

        public MyStack(int sizeStack) {
            maxSize = sizeStack;
            stackArray = new int[maxSize];
            top = -1;
        }

        public void push(int element) {
            stackArray[++top] = element;
            pw.println("ok");
        }

        public int pop() {
            if (top != -1)
                return stackArray[top--];
            else {
                return -999999999;
            }
        }

        public int back() {
            if (top != -1)
                return (stackArray[top]);
            else {
                return -999999999;
            }
        }

        public int size() {
            return top + 1;
        }

        public void clear() {
            top = -1;
            pw.println("ok");
        }

        public void exit() {
            pw.println("bye");
            pw.close();
            System.exit(0);
        }
    }
}