Count the possible sandwiches in a loaf of bread (Racket)

sandwiches.rkt

#lang racket

(require threading)

(define (slice-pairs slice-count)
  (combinations (range slice-count) 2))

;; Any two non-adjacent slices form a sandwich
(define (sandwich-pair? lst)
  (and (list? lst)
       (equal? 2 (length lst))
       (> (abs (apply - lst)) 1)))

(define (possible-sandwiches slice-count)
  (~>> (slice-pairs slice-count)
       (map sandwich-pair?)
       (filter identity)
       length))

;; Or...the math way...
(define (possible-sandwiches-using-math slice-count)
  (~> (- slice-count 2)
      (* (- slice-count 1))
      (/ 2)))

(module+ test
  (require rackunit)
  (for ([func (in-list (list possible-sandwiches
                             possible-sandwiches-using-math))])
    (check-equal? (func 4) 3)
    (check-equal? (func 8) 21)
    (check-equal? (func 20) 171)))