#include <cstring>
#include <iostream>
#include <climits>
using namespace std;
struct Deq {
    int beg = 65536;
    int end = 65536;
    int array[131072] = {};
    void init() {
        for (unsigned i = 0; i < 131072; ++i) {
            array[i] = INT_MIN;
        }
    }
    void PushBack(int n) {
        array[++end] = n;
        cout << "ok" << endl;
    }
    void PushFront(int n) {
        array[--beg] = n;
        cout << "ok" << endl;
    }
    void PopFront() {
        if (end - beg <= 0) {
            cout << "error" << endl;
        } else {
            cout << array[beg] << endl;
            beg++;
        }
    }
    void PopBack() {
        if (end - beg <= 0) {
            cout << "error" << endl;
        } else {
            cout << array[end] << endl;
            end--;
        }
    }
    void Front() {
        if ((end - beg <= 0) || (array[beg] == INT_MIN)) {
            cout << "error" << endl;
        } else {
            cout << array[beg] << endl;
        }
    }
    void Back() {
        if ((end - beg <= 0) || (array[end] == INT_MIN)) {
            cout << "error" << endl;
        } else {
            cout << array[end] << endl;
        }
    }
    void Size() {
        cout << end - beg << endl;
    }
    void Clear() {
        beg = 65536;
        end = 65536;
        for (unsigned i = 0; i < 131072; ++i) {
            array[i] = INT_MIN;
        }
        cout << "ok" << endl;
    }
};
int main() {
    int K;
    cin >> K;
    Deq A;
    A.init();
    int n;
    char b[11] = "push_front";
    char c[10] = "push_back";
    char d[10] = "pop_front";
    char t[9] = "pop_back";
    char x[6] = "front";
    char y[5] = "back";
    char u[5] = "size";
    char e[6] = "clear";
    char r[5] = "exit";
    for (int i = 0; i < K; i++) {
        char *a = new char[15];
        cin >> a;
        if (strcmp(a, b) == 0) {
            cin >> n;
            A.PushFront(n);
        }
        if (strcmp(a, c) == 0) {
            cin >> n;
            A.PushBack(n);
        }
        if (strcmp(a, d) == 0) {
            A.PopFront();
        }
        if (strcmp(a, t) == 0) {
            A.PopBack();
        }
        if (strcmp(a, x) == 0) {
            A.Front();
        }
        if (strcmp(a, y) == 0) {
            A.Back();
        }
        if (strcmp(a, u) == 0) {
            A.Size();
        }
        if (strcmp(a, e) == 0) {
            A.Clear();
        }
        if (strcmp(a, r) == 0) {
            cout << "bye";
            delete[] a;
            break;
        }
        delete[] a;
    }
    return 0;
}