If the numbers 1 to 5 are written out in words: one, two, three, four, five, then there are 3 + 3 + 5 + 4 + 4 = 19 letters used in total.

If all the numbers from 1 to 1000 (one thousand) inclusive were written out in words, how many letters would be used?

NOTE: Do not count spaces or hyphens. For example, 342 (three hundred and forty-two) contains 23 letters and 115 (one hundred and fifteen) contains 20 letters. The use of “and” when writing out numbers is in compliance with British usage. 

import Data.Array.IArray
digit1 ::Array Int String
digit1 =
listArray(,20)["","one","two","three","four","five","six",
"seven","eight","nine","ten","eleven","twelve",
"thirteen","fourteen","fifteen","sixteen",
"seventeen","eighteen","nineteen","twenty"]
digit2 ::Array Int String
digit2 = listArray(,9)["","ten","twenty","thirty","forty","fifty","sixty",
"seventy","eighty","ninety"]
numToWords n | n <21 =digit1!n
| n < 100 = let (a,b) = divMod n 10
in digit2!a++" "++digit1!b
| n < 1000 = let (a,b')= divMod n 100
(b,c) = divMod b' 10
in if b==&&c== then digit1!a++" hundred"
else digit1!a++" hundred and "++numToWords b'
p017 = (+11) .sum $ map (length.filter(/=' ').numToWords) [1..999]