#include <iostream>
#include <vector>
#include <string>
#include <algorithm>
#include <cmath>
#include <set>
#include <numeric>

#define ll long long

using namespace std;


template <typename T>
vector<size_t> sort_indexes(const vector<T> &v) {
    vector<size_t> idx(v.size());
    iota(idx.begin(), idx.end(), 0);
    stable_sort(idx.begin(), idx.end(),
        [&v](size_t i1, size_t i2) {return v[i1].second < v[i2].second;});

    return idx;
}


int main()
{
    ios_base::sync_with_stdio(false);

    ll t;
    cin >> t;
    for (int num = 0; num < t; ++num) {
        ll n, s, e;
        cin >> n;
        vector<pair<int, int>> events(n);
        vector<int> check(n, 0);
        string res;
        for (int i = 0; i < n; ++i) {
            cin >> s >> e;
            events[i] = make_pair(s, e);
        }

        auto idx = sort_indexes(events);
        std::sort(events.begin(), events.end(), [](auto &left, auto &right) {
            return left.second < right.second;
        });


        ll cur_c = -1, cur_j = -1;
        for (int i = 0; i < events.size(); ++i) {
            if (events[i].first >= cur_c) {
                check[i] = 1;
                cur_c = events[i].second;
            } else if (events[i].first >= cur_j) {
                check[i] = 2;
                cur_j = events[i].second;
            }
        }

        cout << "Case #" << num + 1<< ": ";
        if (count(check.begin(), check.end(), 0) > 0) {
            cout << "IMPOSSIBLE";
        } else {
            res.resize(n);
            for (int i = 0; i < n; ++i) {
                if (check[i] == 1)
                    res[idx[i]] = 'J';
                else if (check[i] == 2)
                    res[idx[i]] = 'C';
            }
            cout << res;

        }
        cout << "\n";
    }

    return 0;
}