w

 #include <bits/stdc++.h>

using namespace std;
bool hi(const vector<int> &vec,const vector<int> &vic);
bool hello(const vector<int> &vec, const vector<int> &vic);
int main()
{
    int test;
    cin >> t;
    for (int nin = 1; nin <= test; nin++)
    {
        vector<vector<int> > vec1;
        string re;
        int n, f1 = 0 , f2=0;
        bool w = false;
        cin >> n;



        for (int i = 0; i < n; i++)
        {
            vector<int> vv;

            for (int i = 0; i < 2; i++)
            {
                int x2;
                cin>>x2;
                vv.push_back(x2);
            }
            vv.push_back(0);
            vv.push_back(i);
            vec1.push_back(vv);
        }

        sort(vec1.begin(), vec1.end(), hi);
        for (int k = 0; k < n; k++)
        {


            if (vec1[k][0] >= f2)
            {
                f2 = vec1[k][1];
                vec1[k][2] = 9;
            }else if (vec1[k][0] >= f1)
            {
                f1 = vec1[k][1];
                vec1[k][2] = 6;
            }
            else
            {
                w = true;
            }
        }

        sort(vec1.begin(), vec1.end(), hello);



        if (!w)
        {

            for (int i = 0; i < n; i++)
            {
                if (vec1[i][2] == 6)
                {
                    re = re + 'J';
                }
                else if (vec1[i][2] == 9)
                {
                    re = re + 'C';
                }
            }

            cout <<"Case #"<<nin<<": "<< re << endl;
        }
        else
        {
            cout <<"Case: #"<<nin<<": IMPOSSIBLE"<< endl;

        }
    }



}


bool hi(const vector<int> &vec,const vector<int> &vic)
{
return (vec[0] < vic[0]);
}

bool hello(const vector<int> &vec, const vector<int> &vic)
{
return (vec[3] < vic[3]);
}