uva 11995

#include<stdio.h>
#include<string.h>
#include<math.h>
#include<ctype.h>
#include<stdlib.h>
#include<cstdio>
#include<cstring>
#include<string>
#include<cmath>
#include<iostream>
#include<cctype>
#include<map>
#include<stack>
#include<cstdlib>
#include <queue>
#include <vector>
#include<algorithm>


using namespace std;
typedef long int LD;
typedef long long int LLD;
typedef float F;
typedef double LF;
typedef unsigned int U;
typedef unsigned long int LU;
typedef unsigned long long int LLU;
typedef char C;

#define sf scanf
#define pf printf
#define PI acos(-1.0)
#define sq(x) (x)*(x)
#define MAX(x,y) (x>y)?x:y
#define MIN(x,y) (x<y)?x:y
#define MAX3(x,y,z) (x>y)?((x>z)?x:z):((y>z)?((y>x)?y:x):z)
#define MIN3(x,y,z) (x<y)?((x<z)?x:z):((y<z)?((y<x)?y:x):z)
#define nn printf("\n")
//#define sz 1005

/******   start your code   *******/

int main()
{
    int n,a,b,i,nt,im,q,p,s,coun;
    while(sf("%d",&n)!=EOF)
    {
        queue<int>qu;
        priority_queue<int>pqu;
        stack<int>stk;
        nt=im=q=p=s=coun=0;
        for(i=0; i<n; i++)
        {
            sf("%d%d",&a,&b);
            switch(a)
            {
            case 1:
            {
                qu.push(b);
                pqu.push(b);
                stk.push(b);
            }
            break;
            case 2:
            {
                int x,y,z;
                x=y=z=102;
                coun++;
                if(!qu.empty())
                {
                    x=qu.front();
                    qu.pop();
                }
                if(!pqu.empty())
                {
                    y=pqu.top();
                    pqu.pop();
                }
                if(!stk.empty())
                {
                    z=stk.top();
                    stk.pop();
                }
                if((x==b && y==b)||(x==b && z==b)|| (z==b && y==b))
                    nt++;
                if(x==b)
                    q++;
                if(y==b)
                    p++;
                if(z==b)
                    s++;
                if(x!=b && y!=b && z!=b)
                    im++;
            }
                //break;
            }
        }
        if(nt==coun)
            pf("not sure\n");
        else if(q==coun)
            pf("queue\n");
        else if(p==coun)
            pf("priority queue\n");
        else if(s==coun)
            pf("stack\n");
        else if(im==coun)
            pf("impossible\n");
        else
            pf("impossible\n");
    }
    return 0;
}