#include <iostream>
#include <cstdio>
#include <stack>
#include <queue>
using namespace std;

int main() {
        int n, op, x;
        while (scanf("%d", &n) != EOF) {
                queue<int> q;
                stack<int> s;
                priority_queue<int> pq;
                bool isS = true, isQ = true, isPQ = true;

                for (int i = 0; i < n; i++) {
                        scanf("%d %d", &op, &x);

                        switch (op) {
                        case 1:
                                if (isS)
                                        s.push(x);
                                if (isQ)
                                        q.push(x);
                                if (isPQ)
                                        pq.push(x);
                                break;
                        case 2:
                                if (isS) {
                                        if (s.empty() || s.top() != x)
                                                isS = false;
                                        else
                                                s.pop();
                                }
                                if (isQ) {
                                        if (q.empty() || q.front() != x)
                                                isQ = false;
                                        else
                                                q.pop();
                                }
                                if (isPQ) {
                                        if (pq.empty() || pq.top() != x)
                                                isPQ = false;
                                        else
                                                pq.pop();
                                }
                                break;
                        }
                }

                if (isS == true && isQ == false && isPQ == false)
                        cout << "stack" << endl;
                else if (isS == false && isQ == true && isPQ == false)
                        cout << "queue" << endl;
                else if (isS == false && isQ == false && isPQ == true)
                        cout << "priority queue" << endl;
                else if (isS == false && isQ == false && isPQ == false)
                        cout << "impossible" << endl;
                else
                        cout << "not sure" << endl;
        }
}