//
// Created by heisenberg on 4/4/20.
//

#include <bits/stdc++.h>

#define fi                                      freopen("in.txt", "r", stdin)
#define fo                                      freopen("out.txt", "w", stdout)

#define PII pair<int,int>

using namespace std;

bool overlaps(PII a, PII b) {
    // 1     5
    //    3     6
    if (a.first < b.first && b.first < a.second) {
        return true;
    }
    //    3   6
    //  1   4
    if (a.first < b.second && b.second < a.second) {
        return true;
    }

    if (b.first < a.first && a.first < b.second) {
        return true;
    }

    if (b.first < a.second && a.second < b.second) {
        return true;
    }
    return false;
}

bool overlapExists(const vector<PII > &name, PII x) {
//    for (int i = name.size() - 1; i >= 0; i--) {
    for (int i = 0; i < name.size(); i++) {
        if (overlaps(name[i], x)) {
            return true;
        }
    }
    return false;
}

int main() {
//    fi;
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    int t = 0, T, N;

    cin >> T;


    while (T--) {
        vector<PII > v;

        t++;
        cout << "Case #" << t << ": ";
        int a, b;
        cin >> N;
        for (int i = 0; i < N; i++) {
            cin >> a >> b;
            v.emplace_back(a, b);
        }
        sort(v.begin(), v.end());


        vector<PII > cameron;
        vector<PII > james;
        string ans;

        cameron.push_back(v[0]);
        ans = ans + 'C';
//        cout << v.size() << endl;
        for (int i = 1; i < v.size(); i++) {
            PII x = v[i];

            if (overlapExists(cameron, x)) {
                if (overlapExists(james, x)) {
                    ans = "IMPOSSIBLE";
                    break;
                } else {
                    james.emplace_back(x);
                    ans = ans + 'J';
                }
            } else {
                cameron.emplace_back(x);
                ans = ans + 'C';
            }

//            if (overlapExists(cameron, x) && overlapExists(james, x)) {
//                ans = "IMPOSSIBLE";
//                break;
//            } else if (overlapExists(cameron, x)) {
//                james.emplace_back(x);
//                ans = ans + 'J';
//            } else if (overlapExists(james, x)) {
//                cameron.emplace_back(x);
//                ans = ans + 'C';
//            } else {
//                cameron.emplace_back(x);
//                ans = ans + 'C';
//            }
        }
        cout << ans << endl;

    }

    return 0;
}