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