#include <stdio.h>
#include <vector>
using namespace std;

int main() {
	int iter;
	bool q;
	bool s;
	bool pq;
	int oper;
	int value;
	int max;
	int maxindex;
	vector <int> list;
	while (scanf("%d", &iter) != EOF) {
		max = 0;
		maxindex = -1;
		q = true;
		s = true;
		pq = true;
		printf("_____\n");
		printf("%d\n", list.size());
		for (int i = 0; i < iter; i += 1) {
			scanf("%d %d", &oper, &value);
			if (oper == 1) {
				list.push_back(value);
				if (list.size() == 1) {
					max = list.at(0);
				}
				else {
					if (value > max) {
						max = value;
						maxindex = list.size() - 1;
					}
				}
			}
			else {
				if (list.size() == 0) {
					q = false;
					s = false;
					pq = false;
				}
				else {
					if (list.front() != value) {
						q = false;
					}
					if (list.back() != value) {
						s = false;
					}
					if (max != value) {
						pq = false;
					}
					if (max == value) {
						list.erase(list.begin() + maxindex);
						if (list.size() == 0) {
							max = 0;
							maxindex = -1;
						}
						else {
							max = list.at(0);
							maxindex = 0;
							for (int j = 1; j < list.size(); j += 1) {
								if (list.at(j) > max) {
									max = list.at(j);
									maxindex = j;
								}
							}
						}
					}
					else if (list.at(0) == value) {
						list.erase(list.begin());
						maxindex -= 1;
					}
					else if (list.at(list.size() - 1) == value) {
						list.pop_back();
						maxindex -= 1;
					}
				}
			}
		}
		list.clear();
		if (q == 0 && s == 0 && pq == 0) {
			printf("impossible\n");
		}
		else if (q == 1 && s == 0 && pq == 0) {
			printf("queue\n");
		}
		else if (q == 0 && s == 1 && pq == 0) {
			printf("stack\n");
		}
		else if (q == 0 && s == 0 && pq == 1) {
			printf("priority queue\n");
		}
		else {
			printf("not sure\n");
		}
	}
}