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

public class Queues {
    public static void main(String [] args) throws Exception {
        new Queues();
    }

    BufferedReader in;
    PrintWriter out;
    StringTokenizer st;

    int[] s = new int[2000000];
    int left = 0, right = 0;

    String command = "";

    private String next() throws Exception {
        if (st == null || !st.hasMoreElements())
            st = new StringTokenizer(in.readLine());

        return st.nextToken();
    }
    private int nextInt() throws Exception {
        return Integer.parseInt(next());
    }

    public Queues() throws Exception {
        in = new BufferedReader(new FileReader("input.txt"));
        out = new PrintWriter(new FileWriter("output.txt"));

        solve();
        out.close();
    }

    private void solve() throws Exception {
        while (true) {
            command = next();

            if (command.equals("exit")) {
                out.println("bye");
                break;
            }

            if (command.equals("push")) {
                int val = nextInt();
                push(val);
            } else if (command.equals("pop")) {
                pop();
            } else if (command.equals("front")) {
                front();
            } else if (command.equals("size")) {
                size();
            } else if (command.equals("clear")) {
                clear();
            }
        }
    }

    private void push(int n) {
        s[right] = n;
        right++;
        out.println("ok");
    }

    private void pop() {
        if (right - left >0) {
            int res = s[left];
            left++;
            out.println(res);
        } else {
            out.println("error");
        }
    }

    private void front() {
        if (right - left > 0)
            out.println(s[left]);
        else
            out.println("error");
    }

    private void size() {
        out.println(right - left);
    }

    private void clear() {
        right = left = 0;
        out.println("ok");
    }
}