getSumOfLastN :: (Eq t, Num p, Num t) => [p] -> t -> p
getSumOfLastN [] _ = 0
getSumOfLastN _ 0 = 0
getSumOfLastN x n = last x + getSumOfLastN (init x) (n-1)

--sumLast :: (Num a) => a -> a -> [a]
sumLast el n = helper el n [el]
    where helper a b c = a : (helper newE b (c ++ [newE]))
            where newE = (getSumOfLastN c b)