#include<bits/stdc++.h>
#define pi acos(-1.0)
#define ll int
#define s(n) scanf("%d", &n)

using namespace std;

struct task{
    ll start, end, index, who;
};

/*
1 = c
2 = j
*/

bool crit(task a, task b){
    if(a.start > b.start) return false;
    if(a.start < b.start) return true;
    if(a.end > b.end) return false;
    return true;
}

bool crit2(task a, task b){
    if(a.index > b.index) return false;
    return true;
}

int main()
{
    ll test;
    s(test);
    ll loop;
    for(loop = 1; loop <= test; loop++)
    {
        ll n;
        task t[20000];

        s(n);
        ll i;

        for(i = 1; i <= n; i++){
            s(t[i].start); s(t[i].end);
            t[i].index = i;
        }
        sort(t+1, t+1+n, crit);

        ll oneAss , twoAss = 0;
        bool impossible = false;

        for(i = 1; i <= n && !impossible; i++){
            if(i==1){
                t[i].who = 1;
                oneAss = i;
            }
            else
            {
                if(t[oneAss].end <= t[i].start){
                    t[i].who = 1;
                    oneAss = i;
                }
                else if(twoAss == 0){
                    t[i].who = 2;
                    twoAss = i;
                }
                else if(t[twoAss].end <= t[i].start){
                    t[i].who = 2;
                    twoAss = i;
                }
                else {
                    impossible = true;
                }
            }
        }

        if(impossible) printf("Case #%d: IMPOSSIBLE\n", loop);
        else {
            sort(t+1, t+1+n, crit2);
            printf("Case #%d: ", loop);
            for(i=1; i<=n; i++){
                if(t[i].who == 1) printf("C");
                else printf("J");
            }
            printf("\n");
        }
    }

    return 0;
}