t58

import java.io.*;
import java.util.*;

public class Stacki {
    public static void main(String[] args) throws IOException {
        Stack stack = new Stack(5000);
        BufferedReader in = new BufferedReader(new InputStreamReader (new FileInputStream(new File("input.txt"))));
        StreamTokenizer input = new StreamTokenizer(in);
        PrintWriter out = new PrintWriter(new File("output.txt"));
        while (true) {

            input.nextToken();
            String a = input.sval;
            if (a.equals("push")) {

                input.nextToken();
                stack.push((int) input.nval);
                out.println("ok");
            }
            if (a.equals("front")) {
                if (stack.size()==0){
                    out.println("error");
                } else {
                    out.println(stack.top()) ;
                }
            }
            if (a.equals("size")) {
                out.println(stack.size()) ;
            }
            if (a.equals("clear")) {
                stack.clear() ;
                out.println("ok") ;
            }
            if (a.equals("exit")) {
                out.println("bye") ;
                out.close();
                return;
            }
            if (a.equals("pop")) {
                if (stack.size()==0){
                    out.println("error");
                } else {
                    out.println(stack.pop()) ;
                }
            }
        }
    }
}

class Stack {
    int[] stack;
    int head = 0;
    int tail = 0;
    public Stack(int length){
        stack = new int[length];
    }
    void clear() {
        head = tail;
    }
    int size() {
        return tail-head;
    }
    int top() {
        return stack[head%stack.length] ;
    }
    int pop(){
        head++;
        return stack[(head-1)%stack.length];
    }
    void push(int n) {
        stack[tail%stack.length] = n;
        tail++;
    }
}