10845.cpp

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

int main()
{
	cin.tie(nullptr);
	ios::sync_with_stdio(false);

	int N;
	cin >> N;
	list <int> l;
	for (int i = 0; i < N; i++) {
		string str;
		int num;
		cin >> str;

		if (str == "push") {
			cin >> num;
			l.push_back(num);
		}
		else if (str == "front") {
			if (l.empty())
				cout << "-1" << "\n";
			else
				cout << l.front() << "\n";
		}
		else if (str == "back") {
			if (l.empty())
				cout << "-1" << "\n";
			else
				cout << l.back() << "\n";
		}
		else if (str == "empty") {
			if (l.empty())
				cout << '1' << "\n";
			else
				cout << '0' << "\n";
		}
		else if (str == "pop") {
			if (l.empty())
				cout << "-1" << "\n";
			else
			{
				cout << l.front() << "\n";
				l.pop_front();
			}

		}
		else if (str == "size") {
			cout << l.size() << "\n";
		}
	}
}