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