include <iostream>

include <deque>

include <string>

using namespace std;

class MyDeque { private: deque<int> dq;

public: void push_back(int x) { dq.push_back(x); cout << "ok" << endl; }

void push_front(int x) {
    dq.push_front(x);
    cout << "ok" << endl;
}

int pop_back() {
    if (!dq.empty()) {
        int value = dq.back();
        dq.pop_back();
        cout << value << endl;
        return value;
    } else {
        cout << "error" << endl;
        return -1;
    }
}

int pop_front() {
    if (!dq.empty()) {
        int value = dq.front();
        dq.pop_front();
        cout << value << endl;
        return value;
    } else {
        cout << "error" << endl;
        return -1;
    }
}

int front() {
    if (!dq.empty()) {
        cout << dq.front() << endl;
        return dq.front();
    } else {
        cout << "error" << endl;
        return -1;
    }
}

int back() {
    if (!dq.empty()) {
        cout << dq.back() << endl;
        return dq.back();
    } else {
        cout << "error" << endl;
        return -1;
    }
}

int size() {
    cout << dq.size() << endl;
    return dq.size();
}

void clear() {
    dq.clear();
    cout << "ok" << endl;
}

void exit() {
    cout << "bye" << endl;
}

};

int main() { MyDeque deque;

string command;
int value;

while (cin >> command) {
    if (command == "push_back") {
        cin >> value;
        deque.push_back(value);
    } else if (command == "push_front") {
        cin >> value;
        deque.push_front(value);
    } else if (command == "pop_back") {
        deque.pop_back();
    } else if (command == "pop_front") {
        deque.pop_front();
    } else if (command == "front") {
        deque.front();
    } else if (command == "back") {
        deque.back();
    } else if (command == "size") {
        deque.size();
    } else if (command == "clear") {
        deque.clear();
    } else if (command == "exit") {
        deque.exit();
        break;
    }
}

return 0;

}