#include <bits/stdc++.h>

#define sz(v) (int)v.size()
#define pb push_back

using namespace std;

class Node{
public:
    int value;
    Node* prev, * next;

public:
    Node(int value){
        this->value = value;
        this->prev = this->next = NULL;
    }
};

class LinkedList{
public:
    Node* head, * tail;
public:
    LinkedList(){
        head = tail = nullptr;
    }

    Node* push_front(int value){
        Node* ptr = new Node(value);
        ptr->next = head;
        if(head != nullptr){
            head->prev = ptr;
        }
        if(tail == nullptr){
            tail = ptr;
        }
        head = ptr;
        return ptr;
    }

    Node* push_back(int value){
        Node* ptr = new Node(value);
        ptr->prev = tail;
        if(tail != nullptr){
            tail->next = ptr;
        }
        if(head == nullptr){
            head = ptr;
        }
        tail = ptr;
        return ptr;
    }

    void pop_front(){
        Node* ptr = head->next;
        if(ptr != nullptr){
            ptr->prev = nullptr;
        }else{
            tail = nullptr;
        }
        cout << head->value << '\n';
        delete head;
        head = ptr;
    }

    void pop_back(){
        Node* ptr = tail->prev;
        if(ptr != nullptr){
            ptr->next = nullptr;
        }else{
            head = nullptr;
        }
        cout << tail->value << '\n';
        delete tail;
        tail = ptr;
    }
};

struct Queue{
private:
    LinkedList Node;
    int max_n;
    int size;
public:
    Queue(int n){
        size = 0;
        max_n = n;
    }
    void push_back(int value){
        if(size == max_n){
            cout << "error\n";
        }else{
            Node.push_back(value);
            size++;
        }
    }
    void push_front(int value){
        if(size == max_n){
            cout << "error\n";
        }else{
            Node.push_front(value);
            size++;
        }
    }
    void pop_front(){
        if(size == 0){
            cout << "error\n";
        }else {
            Node.pop_front();
            size--;
        }
    }
    void pop_back(){
        if(size == 0){
            cout << "error\n";
        }else{
            Node.pop_back();
            size--;
        }
    }
};
signed main() {
    ios::sync_with_stdio(false), cin.tie(nullptr), cout.tie(nullptr);
    int n, m;
    string input;
    cin >> n >> m;
    Queue Queue(m);
    while(n--){
        cin >> input;
        if(input == "push_back"){
            int num;
            cin >> num;
            Queue.push_back(num);
        }else if(input == "push_front"){
            int num;
            cin >> num;
            Queue.push_front(num);
        }else if(input == "pop_back"){
            Queue.pop_back();
        }else{
            Queue.pop_front();
        }
    }
}