(define (comb m n)
  (define (citer mm nn prod)
    (if (> mm m) prod
        (citer (+ mm 1) (- nn 1) (/ (* prod nn) mm))))
  (citer 1 n 1))

(define (solve nn)
  (define (addto m n)
    (if (> m n) 0
        (+ (comb m n) (addto (+ m 2) (- n 2)))))
  (+ 1 (addto 2 (- nn 1))))

(display (solve 50))
(newline)