#include <iostream>
#include <string>
using namespace std;

struct Element {
	int value;
	Element* next = 0;
	Element* prev = 0;
};

struct Deque {
	Element* front_ = 0;
	Element* back_ = 0;
	int size_ = 0;
	
	void push_back(int value) {
		Element* new_elem = new Element();
		new_elem->value = value;
		
		if (size_ > 0) {
			back_->next = new_elem;
			new_elem->prev = back_;
		} else {
			front_ = new_elem;
		}
		
		back_ = new_elem;
		size_++;
	}
	
	void push_front(int value) {
		Element* new_elem = new Element();
		new_elem->value = value;
		
		if (size_ > 0) {
			new_elem->next = front_;
			front_->prev = new_elem;
		} else {
			back_ = new_elem;
		}
		
		front_ = new_elem;
		size_++;
	}
	
	int pop_back() {
		int result = back_->value;
		if (size_ > 1) {
			back_ = back_->prev;
			delete back_->next;
			back_->next = 0;
		} else {
			delete back_;
			back_ = 0;
			front_ = 0;
		}
		size_--;
		return result;
	}
	
	int pop_front() {
		int result = front_->value;
		if (size_ > 1) {
			front_ = front_->next;
			delete front_->prev;
			front_->prev = 0;
		} else {
			delete front_;
			back_ = 0;
			front_ = 0;
		}
		size_--;
		return result;
	}
	
	int size() {
		return size_;
	}
	
	void clear() {
		while (size() > 0) {
			pop_front();
		}
	}
	
	int front() {
		return front_->value;
	}
	
	int back() {
		return back_->value;
	}
		
};

int main() {
	Deque deque;
	
	string s;
	cin >> s;
	
	while (s != "exit") {
		if (s == "push") {
			int value;
			cin >> value;
			deque.push_back(value);
			cout << "ok\n";
		} else if (s == "pop") {
			cout << deque.pop_back() << '\n';
		} else if (s == "size") {
			cout << deque.size() << '\n';
		} else if (s == "clear") {
			deque.clear();
			cout << "ok\n";
		} else if (s == "back") {
			cout << deque.back() << "\n";
		}	
		
		cin >> s;
	}
	
	cout << "bye";
	
	return 0;
}