import numpy as np

N = int(input())
Vs = list(map(int, input().split()))

dp = np.full_like(Vs, -1)
for i in range(len(Vs)):
    if i == 0:
        dp[i] = Vs[i]
    elif i == 1:
        dp[i] = max(dp[i-1], Vs[i])
    else:
        dp[i] = max(dp[i-1], dp[i-2] + Vs[i]) 

print(dp[N-1])