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.
This looks to be a bit annoying, as I can't rely on existing libraries (that I know of) and the whole thing is special cases.
...
Well that turned out not to be so bad.
I actually found it faster to write a function that just does the conversion to English and then count the letters than I did to write a function that determined letter counts.