#include<bits/stdc++.h>
#define pii pair<int,int>
#define ff first
#define ss second
#define pb push_back
#define sz 100005
#define sz1 1000005
#define inf 0x3f3f3f3f
#define endl "\n"
 
using namespace std;
 
stack<int>st;
 
queue<int>Q;
 
priority_queue<int>pq;
 
int main()
{
    int a,b,c,i,n,countS,countQ,countpq,counter1;
 
    while(scanf("%d",&n)!=EOF)
    {
        countS=countQ=countpq=0;
 
        counter1 = 0;
 
        for(i=1; i<=n; i++)
        {
            scanf("%d%d",&a,&b);
 
            if(a==2)
            {
                counter1++;
            }
            if(a==1)
            {
                st.push(b);
 
                Q.push(b);
 
                pq.push(b);
            }
            else if(st.size()>0 || Q.size()>0 || pq.size()>0)
            {
 
                if(st.top()==b)
                {
                    countS++;
                    st.pop();
                }
                if(Q.front()==b)
                {
                    countQ++;
                    Q.pop();
                }
                if(pq.top()==b)
                {
                    countpq++;
                    pq.pop();
                }
            }
        }
 
        int cnt =0,tag,tag1,tag2;
 
        tag=tag1=tag2=0;
 
        if(countS==counter1)
        {
            tag=1;
            cnt++;
        }
        if(countQ==counter1)
        {
            tag1=1;
            cnt++;
        }
        if(countpq==counter1)
        {
            tag2=1;
            cnt++;
        }
 
        if(cnt>1)
        {
            printf("not sure\n");
        }
        else if(cnt==1)
        {
            if(tag)
            {
                printf("stack\n");
            }
            else if(tag1)
            {
                printf("queue\n");
            }
            else if(tag2)
            {
                printf("priority queue\n");
            }
        }
        else
        {
            printf("impossible\n");
        }
 
        while(!Q.empty())
        {
            Q.pop();
        }
        while(!st.empty())
        {
            st.pop();
        }
        while(!pq.empty())
        {
            pq.pop();
        }
    }
 
    return 0;
}