Amy uses one checkerboard parity class. Knight moves change parity, so all 200
eligible sites are mutually legal for red stones; before her 100th turn Ben has
made only 99 moves. Ben partitions the board into 100 knight-move 4-cycles and
answers at the opposite vertex. The other two vertices then conflict with
Amy's red vertex, allowing at most one red stone per cycle. Thus the exact
COMPUTED optimum is 100.
