import java.io._
import java.util.StringTokenizer
import scala.util.control.Breaks.{breakable,break}
import scala.collection.mutable.{Map, Stack, Queue, PriorityQueue, Set}

object Main extends App {
  val in = new InputReader(System.in)
  val pw = new PrintWriter(System.out)

  val N = in.next().toInt
  val A = new Array[Int](N+1)
  for (i <- 1 to N)
    A(i) = in.next().toInt

  //
  val dp = Array.ofDim[Long](N+1, 4)
  val init = Int.MinValue * 2L * 1e6.toLong
  for (i <- 0 to N; j <- 0 to 3)
    dp(i)(j) = init

  dp(0)(0) = 0
  for (i <- 0 until N; j <- 0 to 2) {
    if ((i+j) % 2 == 1) { // 使わない
      dp(i+1)(j) = Math.max(dp(i+1)(j), dp(i)(j))
    }
    else {
      dp(i+1)(j+1) = Math.max(dp(i+1)(j+1), dp(i)(j))
      dp(i+1)(j) = Math.max(dp(i+1)(j), dp(i)(j) + A(i+1))
    }
  }

  val k = (N % 2) + 1
  println(dp(N)(k))
}

class InputReader(stream: InputStream) {
  private[this] val reader = new BufferedReader(new InputStreamReader(stream))
  private[this] var tokenizer: StringTokenizer = new StringTokenizer(reader.readLine())
  def next(): String = {
    while (!tokenizer.hasMoreTokens()) {
      tokenizer = new StringTokenizer(reader.readLine())
    }
    tokenizer.nextToken()
  }
}
