#include<iostream>
#include<string>
#include<cstring>
#include<vector>
#include<algorithm>

using namespace std;

struct activity{
    int start;
    int end;
};

bool compareByStart(const activity &a, const activity &b){
    return a.start < b.start;
}


int main(){

    int T;
    cin >> T;
    for(int z = 0; z < T; z++){
        int N;
        cin >> N;
        vector<activity> array;
        string schedule = "";
        int C_end = 0, J_end = 0;
        int not_ok = 1;

        for(int i = 0; i < N; i++){
            activity a;
            cin >> a.start >> a.end;
            array.push_back(a);
        }

        sort(array.begin(), array.end(), compareByStart);

        cout << "Case #" << z + 1 << ": ";
        schedule += "C";
        C_end = array[0].end;
        for(int i = 1; i < array.size(); i++){
            if(array[i].start >= C_end){
                schedule += "C";
                C_end = array[i].end;
            }
            else{
                if(array[i].start >= J_end){
                    schedule += "J";
                    J_end = array[i].end;
                }
                else{
                    not_ok = 0;
                    cout << "IMPOSSIBLE\n";
                    break;
                }
            }
        }

        if(not_ok == 0){
            continue;
        }
        else{
            cout << schedule << "\n";
        }
    }
    return 0;
}