n = int(input())
v = [int(x) for x in input().split()]

res = 0
stack = []
memo = []
for i in range(n):
    memo.append(set())

stack.append(0)
memo[0].add(v[0])
s = v[0]
res = v[0]
end = False
while not end:
    idx = stack[-1]
    v2 = 0
    v3 = 0
    if idx + 2 < len(v):
        v2 = v[idx + 2]
    if idx + 3 < len(v):
        v3 = v[idx + 3]

    if v2 != 0 and s + v2 not in memo[idx + 2]:
        stack.append(idx + 2)
        memo[idx + 2].add(s + v2)
        res = max(s + v2, res)
        s += v2
    elif v3 != 0 and s + v3 not in memo[idx + 3]:
        stack.append(idx + 3)
        memo[idx + 3].add(s + v3)
        res = max(s + v3, res)
        s += v3
    else:
        for j in range(len(stack)):
            vn = v[stack[-1]]
            idx_n = stack.pop()
            if len(stack) < 1:
                if idx_n == 0 and len(v) >= 2:
                    stack.append(1)
                    s = v[1]
                else:
                    end = True
                break
            idx_p = stack[-1]
            s -= vn
            if (s not in memo[idx_p + 2]) or (s not in memo[idx_p + 3]):
                break

print(res)