import java.io.BufferedReader; import java.io.IOException; import java.io.InputStreamReader; import java.util.Collections; import java.util.LinkedList; import java.util.PriorityQueue; import java.util.Queue; import java.util.Stack; import java.util.StringTokenizer; class Main { public static void main (String [] abc) throws IOException { BufferedReader br=new BufferedReader(new InputStreamReader(System.in)); String s; while ((s=br.readLine())!=null) { int opCount=Integer.parseInt(s); Stack<Integer> stk=new Stack<>(); boolean stkOK=true; Queue<Integer> q=new LinkedList<>(); boolean qOK=true; PriorityQueue<Integer> pq=new PriorityQueue<>(Collections.reverseOrder()); boolean pqOK=true; for (int i=0;i<opCount;i++) { StringTokenizer st=new StringTokenizer(br.readLine()); String op=st.nextToken(); int value=Integer.parseInt(st.nextToken()); if (op.equals("1")) { if (stkOK) stk.push(value); if (qOK) q.offer(value); if (pqOK) pq.offer(value); } else if (op.equals("2")) { if (stkOK && (stk.size()==0 || !stk.pop().equals(value))) stkOK=false; if (qOK && (q.size()==0 || !q.poll().equals(value))) qOK=false; if (pqOK && (pq.size()==0 || !pq.poll().equals(value))) pqOK=false; } } if (!stkOK && !qOK && !pqOK) System.out.println("impossible"); else if ((stkOK && qOK) || (stkOK && pqOK) || (qOK && pqOK)) System.out.println("not sure"); else if (stkOK) System.out.println("stack"); else if (qOK) System.out.println("queue"); else if (pqOK) System.out.println("priority queue"); } } }