Shortest-proof-game challenge

Before trying any solutions, I would have to pick the third one, because we know it's going to end with a move by White.

Here's a hint for those who want to try solving Leither's three positions. (#6064)

1st - SPG in 13.0

2nd - SPG in 11.5

3rd - SPG in 13.5

Supposedly, this can be reached without white moving a single piece.

How's this?

Ends in a full move and 23 moves, a prime number! Perfect!!

You need to be more consistent. At #6054 you asked for a proof game in 57.0, but 57 is not a prime number. You of all people should realize that making a proof game in other than a prime number of moves is a violation of common courtesy.

Nice. Is it possible to end with a white move—hang on, is that Endgame outside my window?

#2:

Are you sure these are unique? Because #1 in 13.0 is definitely not.

13 and ends in a full move!

No one claimed these positions have unique SPGs. In fact none of them do.

No, the prime number is just an extra, not a requirement. Ending in a full number of moves is the common decency. In fact the resign button shouldn't even be available in live chess if it's black to move. Black should have to make a move first before resigning.

This legal position was posted by GoodKnightMate00 in the Illegal Position thread. How short a proof game can you find for it? (I've shown the position here with White at the bottom.)

Here's a 26.5 move game. EvinSung will tell us how much shorter it needs to be.

With white to move it might be illegal though.

It's easily legal. Try to make a proof game, as I did at #6079.

How could it be legal with white to move? What would Black's last move have been then?)

Nc3-b1.

It's illegal with white to move in the final position. I just showed how it could be legal if it were black to move.