Let be an even positive integer. We say that two different cells of a board are neighboring if they have a common side. Find the minimal number of cells on he board that must be marked so that any cell marked or not marked) has a marked neighboring cell.
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Let $b$ be an even positive integer. We say that two different cells of a $n \times n$ board are neighboring if they have a common side. Find the minimal number of cells on he $n \times n$ board that must be marked so that any cell marked or not marked) has a marked neighboring cell.
Source: Međunarodna matematička olimpijada, shortlist 1999
Školjka 
be an even positive integer. We say that two different cells of a 
board are neighboring if they have a common side. Find the minimal number of cells on he 
.