10.1.8

import java.util.Scanner;

/**
 * Created by Сергей on 27.05.2016.
 */
public class Sol {
    public static void main(String args[])
    {
        class myStack {
            int arr[];
            int size;
            myStack(){
                arr=new int[100];
                size=0;
            }

            String getCommand(String str){
                int length=str.length();
                int k=1;
                int number=0;
                if (str.charAt(0)=='p' && str.charAt(1)=='u'){
                    for (int i=length-1;i>=5;i--)
                    {
                        number=(str.charAt(i)-'0')*k;
                        k*=10;
                    }
                    arr[size]=number;
                    size++;
                    return "ok";
                }
                if (str.charAt(0)=='p' && str.charAt(1)=='o'){
                    if (size==0) return "error";
                    else {
                        Integer a = arr[size - 1];
                        size--;
                        return a.toString();
                    }
                }
                if (str.charAt(0)=='b' && str.charAt(1)=='a'){
                    if (size==0) return "error";
                    else {
                        Integer a = arr[size - 1];
                        return a.toString();
                    }
                }
                if (str.charAt(0)=='s' && str.charAt(1)=='i'){
                    Integer a=size;
                    return a.toString();
                }
                if (str.charAt(0)=='c' && str.charAt(1)=='l'){
                    size=0;
                    return "ok";
                }
                if (str.charAt(0)=='e' && str.charAt(1)=='x'){
                    return "bye";
                }
                return "WRONG";
            }
        }
        String str;
        Scanner sc=new Scanner(System.in);
        myStack st=new myStack();
        do {
            str=sc.nextLine();
            System.out.println(st.getCommand(str));
        }
        while (str!="exit");
        System.out.println(st.arr[0]);
    }
}