n = int(input())
a = list(map(int, input().split()))

dp = {}

que = [[0, 0]]
def solve():
    while que:
        # print(que, dp)
        i, w = que.pop(0)
        if dp.get(i) and dp[i] != w:
            continue
        if -1 < i - 2 < n: w += a[i - 2]
        for x in range(2, 4):
            ii = i + x
            dp[ii] = max(dp[ii], w) if dp.get(ii) else w
        if i < n:
            que.append([i + 2, w])
            que.append([i + 3, w])
            # dp[i + 2] = w
            # dp[i + 3] = w
solve()
print(max(dp.values()))