sla

#include <bits/stdc++.h>

using namespace std;

int main(){
    int n, x, y;
    stack<int> s;
    queue<int> q;
    priority_queue<int> pq;
    int isq = 1, iss = 1, ispq = 1;

    while(cin >> n){
        isq = 1; iss = 1; ispq = 1;
        while(n--){
            cin >> x >> y;
            if(x){
                s.push(y);
                q.push(y);
                pq.push(y);
            }else if(x == 2){
                if(s.top() != y) iss = 0;
                if(q.front() != y) isq = 0;
                if(pq.top() != y) ispq = 0;

                s.pop();
                q.pop();
                pq.pop();
            }
        }
        if(!isq && !iss && !ispq) cout << "impossible" << endl;
        else if((isq+iss-1) || (isq+ispq-1) || (iss+ispq-1)) cout << "not sure" << endl;
        else if(isq) cout << "queue" << endl;
        else if(iss) cout << "stack" << endl;
        else if(ispq) cout << "priority queue" << endl;
    }
}