(let* ((n (string->number (read-line)))
       (v (list->vector(map string->number (string-split (read-line) " "))))
       (c (make-vector 1001 -1)))
      (letrec ((calc-max 
                   (lambda (i)
                     (if (< 0 (vector-ref c i)) 
                      (vector-ref c i)
                      (cond
                        ((eq? i 1) (let ((e (vector-ref v 0))) (vector-set! c i e) e))
                        ((eq? i 2) (let ((e (max (vector-ref v 0) (vector-ref v 1)))) (vector-set! c i e) e))                       
                        (else (let ((e (max
                                          (calc-max (- i 1)) 
                                          (+ (vector-ref v (- i 1))
                                            (calc-max (- i 2))))))
                                   (vector-set! c i e)
                                   e)))))))
              (print (calc-max n))))