This is not original. I just want to share this problem. If you have heard of it, please give opportunities for those who haven't to answer.
Horst and Queenie are playing a game on a $200\times200$ chessboard. At first, the chessboard was empty. Every move, Horst puts a white knight on an unoccupied cell such that no two knights are attacking each other, then Queenie puts a black Queen on an unoccupied cell. The game ends if someone cannot move. How many knights can be placed a most, no matter the strategy of Queenie?