SP13621 HC12II - Security

ou 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).

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