(this puzzle was taken from an old adventure game)
You are playing on a 4x4 chessboard.
So, columns are a, b, c, d.
Ranks are 1, 2 , 3, 4.
There are in the initial position 2 white Knights on b1, c1, and
2 black knights on b4, c4.
You must interchange the Knights positions (that is, the final position should be
2 white Knights at b4, c4, and 2 black Knights at b1. c1).
During the moves, no piece can be subject to being captured.
White to move.
And a honest advice: This can't be solved in less than 64 moves!
But don't be afraid! Try to solve it!