CodejamQR

def solve(za):
    za = [(za[i][0], za[i][1], i) for i in range(len(za))]

    za.sort(key=lambda x: x[0])

    imran = []

    c_end = 0
    j_end = 0

    for start, end, idx in za:
        if start < c_end and start < j_end:
            return "IMPOSSIBLE"
        elif start >= c_end:
            imran.append(("C", idx))
            c_end = end
        else:  # elif start >= j_end:
            imran.append(("J", idx))
            j_end = end

    imran.sort(key=lambda x: x[1])  # sort according to the idx

    str_imran = ''

    for person, idx in imran:
        str_imran += person

    return str_imran


T = int(input().strip())
list_imran = []

for i in range(1, T+1):
    n = int(input().strip())

    za = []

    for ni in range(n):
        interval = list(map(int, input().strip().split()))

        za.append(interval)

    imran = solve(za)

    list_imran.append(imran)

for index in range(len(list_imran)):
    print("Case #{}: {}".format(index+1, list_imran[index]))