import scala.io.StdIn._
object Main extends App {

  val n: Int = readInt()
  val pointSeq: Seq[(Int, Int)] = Seq.fill(n)(readLine().split(" ").map(_.toInt))
    .map(arr => (arr(0), arr(1)))

  def solve() = {
    val combinations = pointSeq.combinations(2).toSeq
    val totalDistance = combinations.foldLeft(0D) { case (acc, points) =>
      acc + distance(points(0), points(1))
    }
    totalDistance * 2 / n
  }

  def distance(point1: (Int, Int), point2: (Int, Int)): Double = {
    val (x1, y1) = point1
    val (x2, y2) = point2
    Math.sqrt(Math.pow(x1 - x2, 2) + Math.pow(y1 - y2, 2))
  }

  println(solve())

}
