#include <algorithm>
#include <iostream>
#include <queue>
#include <stack>

int main()
{
    int temp{};
    int command{};
    int max{};

    while (std::cin >> max)
        {
            std::queue<int> x_queue{};
            std::priority_queue<int> x_priority_queue{};
            std::stack<int> x_stack{};
            bool isQueue{true}, isPQueue{true}, isStack{true};

            for (int i = 0; i < max; i++)
                {
                    std::cin >> command >> temp;
                    if (command == 1)
                        {
                            x_queue.push(temp);
                            x_priority_queue.push(temp);
                            x_stack.push(temp);
                        }

                    else
                        {
                            if (x_queue.size() == 0)
                                {
                                    isQueue = isPQueue = isStack = false;
                                    continue;
                                }

                            if (temp != x_queue.front()) isQueue = false;

                            if (temp != x_priority_queue.top())
                                isPQueue = false;

                            if (temp != x_stack.top()) isStack = false;

                            x_queue.pop();
                            x_priority_queue.pop();
                            x_stack.pop();
                        }
                }

            int test = isQueue + isPQueue * 2 + isStack * 4;

            switch (test)
                {
                    case 0:
                        std::cout << "impossible" << std::endl;
                        break;
                    case 1:
                        std::cout << "queue" << std::endl;
                        break;
                    case 2:
                        std::cout << "priority queue" << std::endl;
                        break;
                    case 4:
                        std::cout << "stack" << std::endl;
                        break;
                    default:
                        std::cout << "not sure" << std::endl;
                        break;
                }
        }
}