let number_to_words n =
  match n with
  | 1 -> "one" | 2 -> "two" | 3 -> "three" | 4 -> "four" | 5 -> "five"
  | 6 -> "six" | 7 -> "seven" | 8 -> "eight" | 9 -> "nine"
  | 10 -> "ten" | 11 -> "eleven" | 12 -> "twelve"
  | 13 -> "thirteen" | 14 -> "fourteen" | 15 -> "fifteen"
  | 16 -> "sixteen" | 17 -> "seventeen" | 18 -> "eighteen" | 19 -> "nineteen"
  | _ -> ""

let total_letters limit =
  let total = ref 0 in
  for i = 1 to limit do
    total := !total + String.length (number_to_words i)
  done;
  !total

;;

total_letters 19