Erlang solution for Euler problem 17

In project Euler problem number 17 we are asked to determine how many letters would be needed to write all the numbers in words from 1 to 1000?

The following Erlang program solves this in a straightforward manner.

We could have taken some short cuts (we don't actually have to generate the words for each number) but I did it the long way because this is an exercise to learn Erlang.

%% Project Euler, problem 17
%% 
%% What is the sum of the digits of the number 2^1000?

-module(euler17).
-author('Cayle Spandon').

-export([solve/0, solve/1, number_to_string/1, count_letters/1, count_letters_in_number/1]).

-define(SPACE, $\ ).

number_to_string(N) ->
    if
        N == 0 -> "zero";
        N == 1 -> "one";
        N == 2 -> "two";
        N == 3 -> "three";
        N == 4 -> "four";
        N == 5 -> "five";
        N == 6 -> "six";
        N == 7 -> "seven";
        N == 8 -> "eight";
        N == 9 -> "nine";
        N == 10 -> "ten";
        N == 11 -> "eleven";
        N == 12 -> "twelve";
        N == 13 -> "thirteen";
        N == 14 -> "fourteen";
        N == 15 -> "fifteen";
        N == 16 -> "sixteen";
        N == 17 -> "seventeen";
        N == 18 -> "eighteen";
        N == 19 -> "nineteen";
        N == 20 -> "twenty";
        N == 30 -> "thirty";
        N == 40 -> "forty";
        N == 50 -> "fifty";
        N == 60 -> "sixty";
        N == 70 -> "seventy";
        N == 80 -> "eighty";
        N == 90 -> "ninety";
        N < 100 -> number_to_string(N div 10 * 10) ++ "-" ++ number_to_string(N rem 10);
        N < 1000 -> 
            Hundreds = number_to_string(N div 100) ++ " hundred",
            if 
                (N rem 100) == 0 ->
                    Hundreds;
                true ->
                    Hundreds ++ " and " ++ number_to_string(N rem 100)
            end;
        N == 1000 -> "one thousand"
    end.

count_letters([]) ->
    0;
count_letters([H|T]) ->
    if
        (H == ?SPACE) or (H == $-) ->
            count_letters(T);
        true ->
            1 + count_letters(T)
    end.

count_letters_in_number(N) ->
    count_letters(number_to_string(N)).

solve() ->
    solve(1000).

solve(0) ->
    0;
solve(N) ->
    count_letters_in_number(N) + solve(N-1).