9

As you complete the checkmate on the fourth move, a drawer within the chessboard slides open, revealing a slightly smaller chessboard hidden inside, covered in a thick layer of dust. After blowing off the dust, you observe that the design of this board and its pieces appear to originate from an unique era in chess history – a time when checkmate was known, but the concept of check had not yet been invented.

Next to the chessboard, there's a letter. It says: "You just tackled a helpmate in 4. Now, try your hand with this ancient set. On this board, checks that aren't checkmates are illegal. So, play it like yesterday's puzzle, but this time, any check that's not a checkmate is against the rules. Both players still cooperate to achieve checkmate, but without checks. Consequently, a check is also considered checkmate, and therefore legal, if the only responses are non-checkmating checks!"

You are very confused by this last sentence, but luckily you find another chessboard with a position perfectly describing the situation. In this example position, White just played Qf6#, and that's checkmate because the only moves that Black could play to get out of the check are Kd7 and Kxf6, but they are illegal since they would check White without checkmating them.

Now, that was just an example. Here's the problem you need to solve today: Black moves first, then White. Four moves each. White's last move must be checkmate. If you're confused, check out Offerspill's Discord server. Here's the position on Lichess.