object Main extends App {
  val Array(n, k) = io.StdIn.readLine().split(' ').map(_.toInt)
  val pn = io.StdIn.readLine().split(' ').map(_.toInt)
  var max: Double = 0
  for (i <- 0 to n-k) {
    val exp = pn.slice(i, i+k).map(x => (1 to x).sum / x.toDouble).sum
    if (max < exp) max = exp
  }
  println(max)
}