consume :: (Ord a, Num a) => [a] -> a -> a -> a -> a
consume [] _ _ sum = sum
consume xs lh rh sum =
    if lh <= rh then
    consume (tail xs) (max lh next_l) rh (if next_l >= lh then sum else sum + lh - next_l) else
    consume (init xs) lh (max rh next_r) (if next_r >= rh then sum else sum + rh - next_r)
    where next_l = head xs
          next_r = last xs

calculatePuddleCapacity :: (Num p, Ord p) => [p] -> p
calculatePuddleCapacity xs = if length xs <= 2 then 0 else consume (init $ tail xs) (head xs) (last xs) 0

main :: IO ()
main = do
    putStrLn . show $ calculatePuddleCapacity [2, 1, 2]