CF384A Coder

Iahub likes chess very much. He even invented a new chess piece named Coder. A Coder can move (and attack) one square horizontally or vertically. More precisely, if the Coder is located at position $ (x,y) $ , he can move to (or attack) positions $ (x+1,y) $ , $ (x–1,y) $ , $ (x,y+1) $ and $ (x,y–1) $ . Iahub wants to know how many Coders can be placed on an $ n×n $ chessboard, so that no Coder attacks any other Coder.

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