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

#define st first
#define nd second

const int N = 1000 + 5;

int t, n, s, e;
set<pair<pair<int, int>, int>> times;
char ans[N];

int main() {
    cin >> t;
    for(int tt = 1; tt <= t; tt++) {
        cin >> n;
        times.clear();
        for(int i = 1; i <= n; i++){
            cin >> s >> e;
            times.insert({{s, e}, i});
        }

        int outC = -1, outJ = -1;
        int ok = 1;
        for(auto x : times) {
            if(x.st.st < outC and x.st.st < outJ) {
                ok = 0;
                break;
            }
            if(x.st.st < outC) ans[x.nd] = 'J', outJ = x.st.nd;
            else ans[x.nd] = 'C', outC = x.st.nd;
        }
        cout << "Case #" << tt << ": ";
        if(!ok) { cout << "IMPOSSIBLE\n"; continue; }
        for(int i = 1; i <= n; i++) cout << ans[i];
        cout << "\n";
    }
}