N = gets.to_i
V = gets.split.map(&:to_i)

case V.size 
when 1, 2
  puts V.max
  exit
when 3
  puts [V[0]+V[2], V[1]].max
  exit
end

dp = Array.new(N+1){ -1 }
dp[0] = 0
dp[1] = V[0]
dp[2] = V[1]
dp[3] = V[0] + V[2]

4.upto(N) do | i |
  dp[i] = dp[i-3] > dp[i-2] ? dp[i-3] + V[i-1] : dp[i-2] + V[i-1]
end

p dp[-1]