Place three black and three white chess knights
on a 3x4 board as shown in the upper illustration.
The object is to exchange black and white knights in the fewest
possible number of their moves (some examples of such moves are shown
in the bottom diagrams, a and b).
Moves should be made by black and white knights in turn. And after
every move none of the knights should be under attack of any of the
knights of the opposite color. Only one knight can be on a square at
the same time.