import sequtils, strutils

let
  n = parseInt readLine stdin
  vs = stdin.readLine.split.map parseInt

var dp = 0.repeat n
dp[0] = vs[0]

if n >= 2:
  dp[1] = vs[1]
  
if n >= 3:
  dp[2] = vs[0] + vs[2]
  
if n >= 4:
  for i in 3 ..< n:
    dp[i] = vs[i] + max(dp[i.pred 2], dp[i.pred 3])

echo max dp