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

public class Main {
    static PrintWriter out;

    public static class Dequeue {
        static int H, T;
        static int[] arr;

        public Dequeue() {
            H = 26000;
            T = 26000;
            arr = new int[100];
        }

        void pushFront(int n) {
            arr[T % 100] = n;
            T++;
            out.println("ok");
        }

        void pushBack(int n) {
            H--;
            arr[H % 100] = n;
            out.println("ok");
        }

        static void popFront() {
            if (T - H == 0) {
                out.println("error");
                return;
            }
            T--;
            out.println(arr[T % 100]);
        }

        static void popBack() {
            if (T - H == 0) {
                out.println("error");
                return;
            }
            int n = arr[H % 100];
            H++;
            out.println(n);
        }

        static void front() {
            if (T - H == 0) {
                out.println("error");
                return;
            }
            out.println(arr[(T - 1) % 100]);
        }

        static void back() {
            if (T - H == 0) {
                out.println("error");
                return;
            }
            out.println(arr[H % 100]);
        }

        static void size() {
            out.println(Math.abs(T - H));
        }

        static void clear() {
            arr = new int[100];
            H = 26000;
            T = 26000;
            out.println("ok");
        }

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

    public static void main(String[] args) throws FileNotFoundException {
        Scanner in = new Scanner(new File("input.txt"));
        out = new PrintWriter("output.txt");

        Dequeue d = new Dequeue();

        while (in.hasNext()) {
            String m = in.next();
            if (m.equals("push_front")) {
                d.pushFront(in.nextInt());
                continue;
            }
            if (m.equals("push_back")) {
                d.pushBack(in.nextInt());
                continue;
            }
            if (m.equals("pop_front")) {
                d.popFront();
                continue;
            }
            if (m.equals("pop_back")) {
                d.popBack();
                continue;
            }
            if (m.equals("front")) {
                d.front();
                continue;
            }
            if (m.equals("back")) {
                d.back();
                continue;
            }
            if (m.equals("size")) {
                d.size();
                continue;
            }
            if (m.equals("clear")) {
                d.clear();
                continue;
            }
            if (m.equals("exit")) {
                d.exit();
                in.close();
                out.close();
                return;
            }
        }
        in.close();
        out.close();
    }

}