fn integer_length(a: usize) -> usize {

fn digits(a: usize) -> usize {

// Might not work properly if results of `powi`s don't fit into the `usize` casts.
fn integer_range_length(start: usize, end: usize) -> usize {
	let (start_digits, end_digits) = (integer_length(start), integer_length(end));

// Present conversions and type casts could be problematic for certain inputs.
fn range_digits(start: usize, end: usize) -> usize {
	let (start_digits, end_digits) = (digits(start), digits(end));

		(10_f64.powi(start_digits.try_into().unwrap()) - 1.0) as usize,
		10_f64.powi((end_digits - 1).try_into().unwrap()) as usize,

		10_usize.pow(start_digits.try_into().unwrap()) - 1,
		10_usize.pow((end_digits - 1).try_into().unwrap()) as usize,

	(integer_length(a + 1)..=integer_length(b - 1)).fold(0, |acc, elem| {
		acc + 9 * elem * (10_f64.powi((elem - 1).try_into().unwrap()) as usize)

	(digits(a + 1)..=digits(b - 1)).fold(0, |acc, elem| {
		acc + 9 * elem * 10_usize.pow((elem - 1).try_into().unwrap())

							+ integer_range_length(3, s as usize)

							+ range_digits(3, s as usize)