Project Euler Problem 028

zobun :: [Int]
zobun = concatMap(\x->x:[x]) [1..4010]
accum :: [Int]
accum = scanl1 (+) (1:zobun)
nlist :: Int -> [Int]
nlist n = take (2*n) accum
nsum :: Int -> Int
nsum n = (sum $ nlist n)  - 2 - (quot n 2) --うしろの数値は手作業で予想
ans1 :: Int
ans1 = nsum 1001
--669171001
main :: IO ()
main = print ans1