using System;

namespace Stackc
{
    class Program
    {
        static void Main(string[] args)
        {
            int[] stack1 = new int[100000];

            string[] com;
            int first = 49999;
            int last = 50000;
            while (true)
            {
                com = Console.ReadLine().Split();
                if (com[0] == "push_back")
                {
                    stack1[last] = int.Parse(com[1]);
                    last++;
                    Console.WriteLine("ok");
                }
                if (com[0] == "push_front")
                {
                    stack1[first] = int.Parse(com[1]);
                    first--;
                    Console.WriteLine("ok");
                }
                if (com[0] == "pop_front")
                {
                    if (last - first > 1)
                    {
                        first++;
                        Console.WriteLine(stack1[first]);
                    }
                    else Console.WriteLine("error");


                }
                if (com[0] == "pop_back")
                {
                    if (last - first > 1)
                    {
                        last--;
                        Console.WriteLine(stack1[last]);
                    }
                    else Console.WriteLine("error");





                }
                if (com[0] == "front")
                {

                    if (last - first > 1)
                    {
                        Console.WriteLine(stack1[first+1]);
                    }
                    else Console.WriteLine("error");






                }
                if (com[0] == "back")
                {
                    if (last - first > 1)
                    {
                         Console.WriteLine(stack1[last - 1]);
                    }
                    else Console.WriteLine("error");






                }
                if (com[0] == "size")
                {
                    Console.WriteLine(last-first-1);


                }
                if (com[0] == "clear")
                {
                    last = 50000;
                    first = last-1;

                        Console.WriteLine("ok");

                }

                if (com[0] == "exit")
                {
                    Console.WriteLine("bye");
                    break;
                }
            }
        }
    }
}