I Can Guess the Data Structure!(!=EOF!!!)

//#include<bits/stdc++.h>
#include<iostream>
#include<cstdio>
#include<cstring>
#include<string>
#include<set>
#include<vector>
#include<map>
#include<stack>
#include<queue>
#include<cmath>
#include<fstream>
#include<algorithm>
using namespace std;

#define rep(i,a,n) for(int i=a;i<=n;i++)
#define per(i,a,n) for(int i=n;i>=a;i--)
//#define CLR(a) memset(a,sizeof(a),0)

typedef long long ll;
typedef unsigned long long ull;

const int INF = 0x3f3f3f3f;

const int maxn=1e6+10;




stack<int>sta;
queue<int>q;
priority_queue<int>pq;
int main(){
   // ifstream in;
        //ofstream out;
    //in.open("in.txt");
       // out.open("out.txt");
    int n;
    while(scanf("%d", &n!=EOF)){
   // while(in>>n){
        while(!sta.empty())sta.pop();
        while(!q.empty())q.pop();
        while(!pq.empty())pq.pop();
        int f1=1,f2=1,f3=1;
        rep(i,1,n){
            int cmd,x;
            scanf("%d %d", &cmd, &x);
            //in>>cmd>>x;
            if(cmd==1){
                if(f1)sta.push(x);
                if(f2)q.push(x);
                if(f3)pq.push(x);
            }else{
                int t1,t2,t3;
                if(sta.empty())f1=0;
                if(q.empty())f2=0;
                if(pq.empty())f3=0;
                if(f1){
                    t1=sta.top();sta.pop();
                    if(t1!=x)f1=0;
                }
                if(f2){
                    t2=q.front();q.pop();
                    if(t2!=x)f2=0;
                }
                if(f3){
                    t3=pq.top();pq.pop();
                    if(t3!=x)f3=0;
                }
            }
        }
        if(f1+f2+f3>1)puts("not sure");
        else if(f1+f2+f3==0)puts("impossible");
        else if(f1==1){
            puts("stack");
        }else if(f2==1){
            puts("queue");
        }else if(f3==1){
            puts("priority queue");
        }
    }
    return 0;
}