#include <iostream>
#include <string>
#include <vector>
using namespace std;

bool available(int S, int E, vector<vector<int>> tasks) {
    if(tasks.size()==0) return true;
    else {
        for(int i=0; i<tasks.size(); i++) {
            if((S>=tasks[i][0] && S<tasks[i][1]) || (E>tasks[i][0] && E<=tasks[i][1]) || (S<=tasks[i][0] && E>=tasks[i][1])) return false;
        }
        return true;
    }
}

void assignTask(int S, int E, vector<vector<int>> &tasks) {
    if(0<=S && S<=E && E<=1440) {
        tasks.push_back({S, E});
    }
}


void solve() {
    int N;
    cin>>N;

    vector<vector<int>> JTasks, CTasks, time;
    // scheduling time
    for(int i=0; i<N; i++) {
        int S,E;
        cin>>S>>E;
        if(0<=S && S<=E && E<= 1440) {
            time.push_back({S, E});
        }
    }
    // assign tasks
    bool possible=true;
    string str="";
    for(int i=0; i<N; i++) {
        int S=time[i][0];
        int E=time[i][1];
        if(available(S,E,CTasks)) {
            assignTask(S,E,CTasks);
            char c='C';
            str.push_back(c);
            continue;
        } else if(available(S,E,JTasks)) {
            assignTask(S,E,JTasks);
            char j='J';
            str.push_back(j);
            continue;
        } else {
            possible=false;
            break;
        }
    }
    if(possible) cout<<str;
    else cout<<"IMPOSSIBLE";
    cout<<"\n";
}

int main() {
    ios::sync_with_stdio(0);
    cin.tie(0);

    int T, i=1;
    cin>>T;
    while(T--) {
        cout<<"Case #"<<i<<": ";
        solve();
        i++;
    }
}