#include <iostream>

#include <queue>

#include <stack>

using namespace std;

int main()

{

	ios::sync_with_stdio(false);

	cin.tie(nullptr);

	cout.tie(nullptr);

	int n, in, x;

	while (cin >> n) {

		queue<int> q;

		priority_queue<int> pq;

		stack<int> s;

		bool isq = true, ispq = true, iss = true;

		while (n--) {

			cin >> in >> x;

			if (in == 1) {

				q.emplace(x);

				pq.emplace(x);

				s.emplace(x);

			}

			else {

				if (isq)

					if (!q.empty() && q.front() == x) q.pop();

					else isq = false;

				if (ispq)

					if (!pq.empty() && pq.top() == x) pq.pop();

					else ispq = false;

				if (iss)

					if (!s.empty() && s.top() == x) s.pop();

					else iss = false;

			}

		}

		if ((isq && iss) || (isq && ispq) || (ispq && iss)) cout << "not sure\n";

		else if (isq) cout << "queue\n";

		else if (ispq) cout << "priority queue\n";

		else if (iss) cout << "stack\n";

		else cout << "impossible\n";

	}

	return 0;

}