(define n (string->number (read-line)))
(define v (list->vector(map string->number (string-split (read-line) " ")))) 
(define (calc-max i a1 a2)
        (let1 m (max
          a1 
          (+ (vector-ref v (+ (- n i) 2)) a2))
          (if (= i 3) m
                        (calc-max (- i 1) m a1))))
(define v0 (vector-ref v 0))                  
(print (cond ((= n 1) v0)
              ((= n 2) (max v0 (vector-ref v 1)))
              (else 
                (calc-max n 
                          (max v0 (vector-ref v 1))        
                          v0))))