Sort:  
  1. ... Rxa2+
  2. Kxa2 c1N+
  3. Ka1 Nxb3+
  4. Ka2 ???
    Please continue!

Ra4xa2+, Ka1xRa2, c2-c1N+

The white king is in a2 now so,

Ka2-a1, Nc3xb3+, Ka1-a2, Nb3-c1+
Ka2-a1, Bc3x b2 mate

Correct, but what after:

  1. ... Rxa2+
  2. Kxa2 c1N+
  3. Ka1 Nxb3+
  4. Ka2 Nc1+
  5. Ka3 ???

I announced you as winner together with @gaby-de-wilde and @afrog .

Qd8-f8+

Hint: look at your rook ...

Hey @pompe72, you've got it already! Why you doesn't finish your good game?

  1. ... Rxa2+
  2. Kxa2 c1N+
  3. Ka1 Nxb3+
  4. Ka2 Nc1+
  5. Ka3 why you stopped here @pompe72? Just two steps to the end Ra8+
  6. Bxa1 Qxa8+ finish!

@pompe72 made the game @jaki01. Not me!

I announced you as winner together with @pompe72 and @gaby-de-wilde.

Yes, this time @pompe72 made most of the job. :)

Cause I'm tired.

Ok, that we understand. Have a nap and then steem on!

5 . ... Qf8+ wins, too, but it is not the fastest way (white can delay the checkmate with 6. Qd6).
What is the fastest way to win for black after 5. Ka3?

Must be then
Rb8-b3+ , Ka4
Qd7+ mate

No, Qd7+ is not checkmate (white bishop). :-)

Q takes B mate