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main = print . sum . take 20 . map tetra . iterate ((+ 9) . (* 10)) $ 9
where tetra n = (n * (n + 1) * (n + 2)) `div` 6
main = print . sum . take 20 . map tetra . iterate ((+ 9) . (* 10)) $ 9
where tetra n = (n * (n + 1) * (n + 2)) `div` 6