Enumerative combinatorics
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The sum of the areas of the parallelogram polyominoes having semi-perimeter n + 2 is equal to 4n. In this paper we give a simple proof of this property by means of a mapping from the cells of parallelogram polyominoes having semiperimeter n + 2 to the 4n words of length n of the free monoid {a, b, c, d}*. This mapping works in linear time. Then, we introduce a tiling game arising from this enumerative property.