(define n-list-sum
  (lambda (nlist)
    (cond [(null? nlist) 0]
          [(number? (car nlist))
           (+ (car nlist)
              (n-list-sum (cdr nlist)))]
          [else 
           (+ (n-list-sum (car nlist))
              (n-list-sum (cdr nlist)))])))