# frozen_string_literal: true

class Array
  def homura_point
    pairs = each_slice(2)
    
    left_sums = Array.new(N + 1, 0)
    right_sums = Array.new(N + 1, 0)

    pairs.each_with_index do |pair, index|
      left = pair[0]
      left_sums[index + 1] = left_sums[index] + left
    end

    pairs.reverse_each.with_index do |pair, index|
      right = pair[1]
      right_sums[N - index - 1] = right_sums[N - index] + right
    end

    left_sums.zip(right_sums).map { _1 + _2 }.max
  end
end

def solve
  2 * A.homura_point - A.sum
end

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

puts solve