t61

import java.io.File;
import java.io.*;
import java.util.*;
public class Deque {
    static int head = 26000;
    static int tail = 26000;
    static int[] a = new int[100];
    static PrintWriter out;
    static void push_front(int n) {
        a[tail % 100] = n;
        tail++;
        out.println("ok");
    }
    static void push_back(int n) {
        head--;
        a[head % 100] = n;
        out.println("ok");
    }
    public static void pop_front() {
        if(tail- head == 0) {
            out.println("error");
            return;
        }
        tail--;
        out.println(a[tail % 100]);
    }
    public static void pop_back() {
        if(tail - head == 0) {
            out.println("error");
            return;
        }
        int n = a[head % 100];
        head++;
        out.println(n);
    }
    public static void front() {
        if(tail - head == 0) {
            out.println("error");
            return;
        }
        out.println(a[(tail - 1) % 100]);
    }
    public static void back() {
        if(tail - head == 0) {
            out.println("error");
            return;
        }
        out.println(a[head % 100]);
    }
    public static void size() {
         out.println(Math.abs(tail - head));
    }
    public static void clear() {
        a = new int[100];
        head = 26000;
        tail = 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")) {
                push_front(in.nextInt());
                continue;
            }
            if (m.equals("push_back")) {
                push_back(in.nextInt());
                continue;
            }
            if (m.equals("pop_front")) {
                pop_front();
                continue;
            }
            if (m.equals("pop_back")) {
                pop_back();
                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();
    }

}