from collections import deque
N = int(input())
V = list(map(int,input().split()))
pq = deque()
pq.append([0,V[0]])
if N > 1:
  pq.append([1,V[1]])
max = V[0]
while len(pq):
  n = pq.popleft()
  for i in range(n[0]+2,N):
    c = n[1] + V[i]
    if i + 2 < N:
      pq.append([i,c])
    else:
      if max < c:
        max = c
print(max)