def main()
  n = gets.to_i
  str = gets.to_s
  if n == 1 then
    p str.to_i
    return
  end
  str = str.split(" ")
  sum = 0

  v = []
  str.each { |num|
    v << num.to_i
  }


  if maxLeft(v[0], v[1]) then
    index = 0
  else
    index = 1
  end

  sum = v[index]

  loop {
    if index+2 >= n then
      break
    elsif index+3 >= n then
      sum += v[index+1]
      break
    elsif maxLeft(v[index+2], v[index+3]) then
      index += 2
    else
      index += 3
    end
    sum += v[index]
  }

  p sum
end

def maxLeft(a, b)
  return a > b ? true : false
end

main()