SECURITY (35 points)

You are designing a new encryption system that works in the following way:

For server-client communication you need a key k, composed of m sections, each
of length l, and the key consists only of lowercase characters in the set {a, b,
c, d, e, f}. The server has a key k1 and the client has a key k2 where:

k1 = f(k). f is a function that receives a key and replace some random letters
by ? indicating that those characters can be any lowercase letter of the set
described before.

k2 = f(g(k)). g is a function that takes a key and produces a random permutation
of its m sections. And f is the function defined above.


For example: let m = 3, l = 2

f('abacbc') = '?ba??c'
g('abacbc') = 'acbcab' (each section was moved one place to the left).

Your task is given k1 and k2, find key k. If there are several solutions, print
the lexicographically smallest key. And if there is no solution at all, print
"IMPOSSIBLE" (without the quotes).


INPUT DESCRIPTION

The first line has a single integer T, which corresponds to the number of test
cases. T test cases follows: the first line of the test case corresponds to the
integer m, the second line contains the string k1 and the third line contains
the string k2.


CONSTRAINTS

T <= 20
0 < |k1| <= 100
0 < m <= 50
|k2| = |k1|
It is guaranteed that m is always a divisor of |k1|
k1 and k2 consist of {a, b, c, d, e, f, ?}


OUTPUT DESCRIPTION

For test case i, numbered from 1 to T, output "Case #i: ", followed by the
lexicographically smallest key or "IMPOSSIBLE".


EXAMPLE INPUT

    5
    2
    abcd
    c?ab
    3
    ab?c?c
    ac?c??
    3
    ab?c?c
    aabbdd
    2
    aa
    bb
    2
    abcd
    cdab


EXAMPLE OUTPUT

    Case #1: abcd
    Case #2: abacac
    Case #3: IMPOSSIBLE
    Case #4: IMPOSSIBLE
    Case #5: abcd