#include <bits/stdc++.h>
using namespace std;

#define INF INT_MAX
#define LINF LLONG_MAX
#define forin(data) for(auto &i : data)
#define debug(x) cerr << #x << " = " << x << endl;

using ll = long long;
using pi = pair<int, int>;
using vi = vector<int>;
using vii = vector<vi>;
using vl = vector<ll>;

void init(){
    #ifdef LOCAL
        freopen("input.txt", "r", stdin);
        freopen("output.txt", "w", stdout);
        freopen("log.txt", "w", stderr);
    #else
        cin.tie(0); cout.tie(0);
        ios_base::sync_with_stdio(0);
    #endif
}

bool sortStartTime(vi &v1, vi &v2){
    return v1[0] < v2[0];
}

bool sortIndex(vi &v1, vi &v2){
    return v1[2] < v2[2];
}

int main(){
	init()
    int tc;
    cin >> tc;
    for(int i=1; i<=tc; i++){
        bool check = true;
        int n, index = 0;
        cin >> n;
        string ans = "";
        vii arr(n, vi(4));
        for(int i=0; i<n; i++){
            cin >> arr[i][0] >> arr[i][1];
            arr[i][2] = i;
        }
        sort(arr.begin(), arr.end(), sortStartTime);
        int j, c;
        j = c = -1;
        for(int i=0; i<n; i++){
            if(arr[i][0] >= j){
                j = -1;
            }
            if(arr[i][0] >= c){
                c = -1;
            }
            if(j != -1 && c != -1){
                check = false;
                break;
            }
            if(j == -1){
                j = arr[i][1];
                arr[i][3] = 0;
                //ans += "J";
                //cout << "J Assigned! Cost: " << j << endl;
            }else{
                c = arr[i][1];
                arr[i][3] = 1;
                //ans += "C";
                //cout << "C Assigned! Cost: " << c << endl;
            }
        }
        sort(arr.begin(), arr.end(), sortIndex);
        if(!check){
            ans = "IMPOSSIBLE";
        }else{
            for(int i=0; i<n; i++){
                ans += (arr[i][3] == 0 ? "J" : "C");
            }
        }
        cout << "Case #" << i << ": " << ans << endl;
    }
    return 0;
}