n = int(input())
a = list(map(int, input().split()))
dp = [0] * n
dp[0] = a[0]
dp[2] = dp[0] + a[2]

for i in range(3, n):
    dp[i] = max(dp[i - 2], dp[i - 3]) + a[i]
print(dp[n - 1])