17

The cold North Pole air is crisp as you and the group of elves start your journey towards Santa's Workshop. Some elves begin singing Christmas carols, all led by front singer Magnus Carolsen.

Halfway through your path, a massive snowdrift blocks the way. It's clear that this is no ordinary obstacle - the snow seems to have been deliberately piled up, obstructing the only known route to Santa's Workshop.

As the group discusses how to proceed, you notice something unusual about the formation of the snow. It almost resembles a chessboard pattern. You approach the snowdrift and discover a series of chess pieces carefully placed on the top layer of snow, forming chess problem.

Magnus Carolsen looks over your shoulder and sings, "This must be Grinchuk! He's trying to slow us down." He turns to you, "We need your expertise again. Can you solve this chess problem and find a way through?"

The elves gather around as you study the chess pieces. The pieces are structured in a strange way, you find it almost impossible to see how this position could occur. You then see something written in the snow: "It's Black to move. What was the last four half-moves played?"

In this chess position, it's Black's turn to move. This position could have happened in only one specific way. Can you figure out the last four half-moves? That includes two moves by White and two by Black. Please list the four moves in order, starting from the first move in this sequence and ending with the current position. Here's a board editor, and here's Offerspill's Discord server if you need help understanding the problem.