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

#define FASTIO ios_base::sync_with_stdio(false), cin.tie(0)

int main() {
  FASTIO;
  int t;
  cin >> t;
  for (int i = 1; i <= t; i++) {
    int n;
    cin >> n;
    vector<pair<int, int>> h;
    for (int j = 0; j < n; j++) {
      int x, y;
      cin >> x >> y;
      h.push_back(make_pair(x, y));
    }
    vector<pair<int, int>> aux = h;
    sort(h.begin(), h.end());
    string ans = "C";
    int c = h[0].second, j = -1;
    for (int k = 1; k < n; k++) {
      if (h[k].first >= j) {
        ans += "J";
        j = h[k].second;
      }
      else if (h[k].first >= c) {
        ans += "C";
        c = h[k].second;
      }
      else {
        ans = "IMPOSSIBLE";
        break;
      }
    }
    if (ans != "IMPOSSIBLE") {
      string now = "";
      for (int k = 0; k < n; k++) {
        for (int z = 0; z < n; z++) {
          if (h[z] == aux[k]) {
            now += ans[z];
            break;
          }
        }
      }
      cout << "Case #" << i << ": " << now << endl;
    }
    else {
      cout << "Case #" << i << ": " << ans << endl;
    }
  }
  return 0;
}