inputs = STDIN.readlines.map(&:chomp)
N = inputs[0].to_i
Vs = inputs[1].split(/\s+/).map(&:to_i)

class Block
  def initialize(range, head_throughs, tail_throughs, max_value)
    @range = range
    @head_throughs = head_throughs
    @tail_throughs = tail_throughs
    @max_value = max_value
  end

  attr_reader :range, :head_throughs, :tail_throughs, :max_value
end

def merge(blocks1, blocks2)
  blocks1.product(blocks2).select { |block1, block2|
    throughs = block1.tail_throughs + block2.head_throughs
    1 <= throughs && throughs <= 2
  }.map { |block1, block2|
    Block.new(
      block1.range.first..block2.range.last,
      block1.head_throughs,
      block2.tail_throughs,
      block1.max_value + block2.max_value
    )
  }.group_by { |block|
    [block.head_throughs, block.tail_throughs]
  }.map { |throughs, blocks|
    blocks.max_by(&:max_value)
  }
end

cands = Vs.map.with_index { |v, i|
  [Block.new(i..i, 0, 0, v), Block.new(i..i, 1, 1, 0)]
}

until cands.size == 1
  cands = cands.each_slice(2).map { |blocks1, blocks2|
    if blocks2
      merge(blocks1, blocks2)
    else
      blocks1
    end
  }
end

puts cands.first.max_by(&:max_value).max_value