#include <bits/stdc++.h>

using namespace std;

class Tasks{
    public:
        int start;
        int end;
        string jorc = "C";
        int originalPlace = 0;
        Tasks(){
            start = 0;
            end = 0;
        }
        Tasks(int start, int end){
            start = start;
            end = end;

        }
};

// Sort Tasks from earliest to latest;
void sortTasks(Tasks arr[], int n)
{
    int i;
    for(i = 0; i < n; i++)
    {
        int j;
        for(j = 0; j < n; j++)
        {
            if(arr[i].start < arr[j].start)
            {
                Tasks temp;
                temp = arr[i];
                arr[i] = arr[j];
                arr[j] = temp;
            }
        }
    }
}

// Sort Tasks based on Original place;
void revertSortTasks(Tasks arr[], int n)
{
    for(int i = 0; i < n; i++)
    {
        for(int j = 0; j < n; j++)
        {
            if(arr[i].originalPlace < arr[j].originalPlace)
            {
                Tasks temp;
                temp = arr[i];
                arr[i] = arr[j];
                arr[j] = temp;
            }
        }
    }
}

int main(){

    int T;
    scanf("%d", &T);

    string results[T];

    for(int i = 0; i < T; i++){
        int howMany;
        scanf("%d", &howMany);

        Tasks taskArray[howMany];

        bool possible = true;

        for(int j = 0; j < howMany; j++){
            scanf("%d" "%d", &taskArray[j].start, &taskArray[j].end);
            taskArray[j].originalPlace = j;
        }
        //sort as task por ordem de inicio;
        sortTasks(taskArray, howMany);

        //setjorc(taskArray);
        for (int j = 0; j < howMany; j++){
            int overlaps = 0;
            for (Tasks other : taskArray){
                if (taskArray[j].start > other.start && other.end > taskArray[j].start){
                    overlaps++;
                    if (other.jorc == "C"){
                        taskArray[j].jorc = "J";
                    }
                }
                if(overlaps >= 2){
                    possible = false;
                    break;
                }
            }
        }

        //revert o sort;
        revertSortTasks(taskArray, howMany);

        //saves results;
        if (possible){
            string str = taskArray[0].jorc;
            for (int j = 1; j < howMany; j++){
                str = str + taskArray[j].jorc;
            }
            results[i] = str;
        }
        else{
            results[i] = "IMPOSSIBLE";
        }
    }

    for(int i = 0; i < T; i++){
        int caso = i + 1;
        printf("Case #%d: %s\n", caso, results[i].c_str());
    }
    return 0;
}