#include <bits/stdc++.h>
using namespace std;
int main() {
int n,a,b;
int q,s,p;
while(cin>>n){
queue<int> Q;
stack<int> S;
priority_queue<int> P;
q=1;
s=2;
p=3;
while(n--){
cin>>a>>b;
if(a==1){
Q.push(b);
S.push(b);
P.push(b);
}
else if(a==2){
if(!Q.empty()&&Q.front()==b)Q.pop();
else q=0;
if(!S.empty()&&S.top()==b)S.pop();
else s=0;
if(!P.empty()&&P.top()==b)P.pop();
else p=0;
}
}
int ans=max( max(q,p) , s);
if(!ans)cout<<"impossible"<<endl;
else if((q&&p)||(q&&s)||(s&&p)||(q&&p&&s))cout<<"not sure"<<endl;
else if(q)cout<<"queue"<<endl;
else if(s)cout<<"stack"<<endl;
else if(p)cout<<"priority queue"<<endl;
}
return 0;
}
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