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


public class Main {

    static PrintWriter out;
    static BufferedReader br;
    static StringTokenizer in;

    public static void main(String[] args) throws IOException {
        br = new BufferedReader(new InputStreamReader(System.in));
        out = new PrintWriter(System.out);


        MyDek dek = new MyDek(2000000);
        String command;
        int s;
        do {
            command = nextToken();
            switch (command) {
                case "push_front":
                    dek.push_front(nextInt());
                    break;
                case "push_back":
                    dek.push_back(nextInt());
                    break;
                case "pop_back":
                    s = dek.pop_back(dek.size());
                    out.println((s == -999999999) ? ("error") : (s));
                    break;
                case "pop_front":
                    s = dek.pop_front(dek.size());
                    out.println((s == -999999999) ? ("error") : (s));
                    break;
                case "front":
                    out.println((dek.front(dek.size()) == -999999999) ? ("error") : dek.front(dek.size()));
                    break;
                case "back":
                    out.println((dek.back(dek.size()) == -999999999) ? ("error") : dek.back(dek.size()));
                    break;
                case "size":
                    out.println(dek.size());
                    break;
                case "clear":
                    dek.clear();
                    break;
                default:
                    dek.exit();
            }
        } while (command.equals("exit") != true);
        out.close();
        System.exit(0);


    }

    static class MyDek {
        int maxSize;
        int[] dekArray;
        int front;
        int rear;
        int nItems;

        public MyDek(int sizeDek) {
            maxSize = sizeDek;
            dekArray = new int[maxSize];
            front = maxSize / 2 + 1;
            rear = maxSize / 2;
            nItems = 0;
        }

        public void push_front(int element) {
            dekArray[--front] = element;
            nItems++;
            out.println("ok");
        }

        public void push_back(int element) {
            dekArray[++rear] = element;
            nItems++;
            out.println("ok");
        }

        public int pop_back(int size) {
            if (size != 0) {
                nItems--;
                return dekArray[rear--];
            } else
                return -999999999;
        }

        public int pop_front(int size) {
            if (size != 0) {
                int temp = dekArray[front++];
                nItems--;
                return temp;
            } else
                return -999999999;
        }

        public int back(int size) {
            if (size != 0)
                return (dekArray[rear]);
            else
                return -999999999;
        }

        public int front(int size) {
            if (size != 0)
                return (dekArray[front]);
            else
                return -999999999;
        }

        public int size() {
            return nItems;
        }

        public void clear() {
            front = maxSize / 2 + 1;
            rear = maxSize / 2;
            nItems = 0;
            out.println("ok");
        }

        public void exit() {
            out.println("bye");
        }

    }

    public static String nextToken() throws IOException {
        while (in == null || !in.hasMoreTokens()) {
            in = new StringTokenizer(br.readLine());
        }
        return in.nextToken();
    }

    public static int nextInt() throws IOException {
        return Integer.parseInt(nextToken());
    }

}