(defun masses-from-file (filepath)
  (with-open-file (handle filepath)
                  (loop for line = (read-line handle nil nil)
                        while line
                        collect (parse-integer line))))

(defun fuel-for-mass (n)
  (- (floor (/ n 3))
     2))

(defun total-fuel-for-mass (a b)
  (cond
   ((< (fuel-for-mass b) 0) 0)
   (t (+ (+ a (fuel-for-mass b))
         (total-fuel-for-mass 0 (fuel-for-mass b))))))

(write (reduce #'total-fuel-for-mass
               (masses-from-file "../input")
               :initial-value 0))