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

public class task61 {
    public static class Deqeue {
        static int H = 26000;
        static int T = 26000;
        static int[] arr = new int[100];
        static PrintWriter out;

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

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

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

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

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

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

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

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

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

}