ProjectEuler 2

; ProjectEuler.net
;; Even Fibonacci numbers
;; Problem 2
;; Each new term in the Fibonacci sequence is generated by adding the previous two terms. By starting with 1 and 2, the first 10 terms will be:
;;
;; 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, ...
;;
;; By considering the terms in the Fibonacci sequence whose values do not exceed four million, find the sum of the even-valued terms.

(defun fsequence (limit)
  (do ((x 1 y)
       (y 2 (+ x y))
       (even-sum 0 (if (evenp y) (+ even-sum y) even-sum))
       (sum 1 (+ sum y))
       (f-list (list 1) (push y f-list)))
      ((>= y (1- limit)) even-sum)))

(defun answer ()
  (fsequence 4000000)) ; 4613732