(let* ((n (string->number (read-line)))
       (v (list->vector(map string->number (string-split (read-line) " ")))))
      (letrec ((calc-max 
                   (lambda (i a1 a2)
                     (cond
                       ((eq? i 3) (max
                                    a1 
                                    (+ (vector-ref v (- i 1)) a2))) 
                       (else (calc-max 
                                (- i 1)
                                (max
                                  a1 
                                  (+ (vector-ref v (- i 1)) a2))
                                a1))))))
              (print (cond ((eq? n 1) (vector-ref v 0))
                           ((eq? n 2) (max (vector-ref v 0) (vector-ref v 1)))
                            (else 
                              (calc-max n 
                                        (vector-ref v 0)
                                        (max (vector-ref v 0) (vector-ref v 1))))))))