#include <bits/stdc++.h>
using namespace std;
#pragma GCC optimize("-Ofast")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,sse4.2,popcnt,abm,mmx,avx2,tune=native")
#pragma GCC optimize("-ffast-math")
#pragma GCC optimize("-funroll-loops")
#pragma GCC optimize("-funroll-all-loops,-fpeel-loops,-funswitch-loops")
#define IO ios_base::sync_with_stdio(0), cin.tie(0), cout.tie(0);
const long long MOD = 1e9 + 7, OO = 1e18;
const double PI = acos(-1);
const int N = 1e6 + 5;
const int dx[4] = {0, 0, 1, -1};
const int dy[4] = {1, -1, 0, 0};

// struct cmp
// {
//     bool operator() (pair<long long, long long> a, pair<long long, long long>b)
//     {
//         if (a.first != b.first)return a.first < b.first;
//         return a.second > b.second;
//     }
// };

long long n, t, x;

int main()
{
    IO
    // set<long long, greater<long long>>s;
    // s.insert(1);
    // s.insert(2);
    // for (auto x : s)cout << x << '\n';

    // priority_queue<pair<long long, long long>>pq;
    // // {5, 5}, {5, 6}
    // pq.push({5, 5});
    // pq.push({5, 6});
    // pq.push({4, 4});
    // while (!pq.empty())
    // {
    //     cout << pq.top().first << ' ' << pq.top().second << '\n';
    //     pq.pop();
    // }
    while (cin >> n)
    {
        stack<long long>st;
        queue<long long>q;
        priority_queue<long long>pq;
        long long cnt1 = 0, cnt2 = 0;
        while (n--)
        {
            cin >> t >> x;
            if (t == 1)
            {
                cnt1++;
                st.push(x);
                q.push(x);
                pq.push(x);
            }
            else
            {
                cnt2++;
                if (st.size() && st.top() == x)st.pop();
                if (q.size() && q.front() == x)q.pop();
                if (pq.size() && pq.top() == x)pq.pop();
            }
        }
        long long cnt = 0, ans;
        if (st.size() == cnt1 - cnt2)
        {
            cnt++;
            ans = 1;
        }
        if (q.size() == cnt1 - cnt2)
        {
            cnt++;
            ans = 2;
        }
        if (pq.size() == cnt1 - cnt2)
        {
            cnt++;
            ans = 3;
        }
        if (cnt == 0)cout << "impossible\n";
        else if (cnt > 1)cout << "not sure\n";
        else if (ans == 1)cout << "stack\n";
        else if (ans == 2)cout << "queue\n";
        else cout << "priority queue\n";
    }
    return 0;
}

/*
    fifo
    1 2 3 4 5
    push => O(1);
    pop => O(1);
    front => O(1);
    size
    empty

    push
    pop
    top
    size
    empty

    4 5 8
    push o(long)
    pop o(log)
    top o(1)


*/