$n = gets.to_i - 1
$v = gets.split.map(&:to_i)

def dfs(n, memo)
  return $v[0] if n==0
  return $v[1] if n==1

  tmp = -1
  (n-2).downto(0) do |i|
    tmp = [tmp, (memo[i] < 0 ? dfs(i, memo) : memo[i]) ].max
  end

  memo[n] = tmp + $v[n]
  return memo[n]
end

puts dfs($n, Array.new($n+1, -1))