(define (cumb N C) (let r ((n 0) (t 1) (s 1))
  (if (= n N) (/ s (expt 2.0 C))
    (let* ((np (+ n 1)) (nt (/ (* t (- C n)) np))) (r np nt (+ s nt))))))

(cumb 142 332) ; => 0.004895195039177136