object Hon1 extends Application {
  //Console.setIn(new java.io.FileReader("input.txt"))
  val ary = Console.readLine().split(" ")
  val n = Integer.parseInt(ary(0))
  val k = Integer.parseInt(ary(1))
  val a = List.tabulate(n, i => Console.readInt).toArray
  
  val firstSum = a.take(k).reduceLeft((i:int,j:int) => i + j)
  def calc(i:int, prevSum:int, maxSum:int):int = {
    if(i == n) return maxSum
    val curSum = prevSum - a(i - k) + a(i) 
    return calc(i + 1, curSum, Math.max(maxSum, curSum)) 
  }
  
  println(calc(k, firstSum, firstSum))
}