import java.util.Scanner

object Main extends App {

  val sc = new Scanner(System.in)

  val n = sc.nextInt
  val ps = Seq.fill(n)((sc.nextInt, sc.nextInt))

  val total = ps.combinations(2).foldLeft(0.0d){ case (sum, Seq(p1, p2)) =>
      sum + distance(p1, p2)
  }

  val res = total * 2 / n

  println(res)

  def distance(p1: (Int, Int), p2: (Int, Int)): Double = {
    val (x1, y1) = p1
    val (x2, y2) = p2
    Math.sqrt(( x1 - x2) * ( x1 - x2) + ( y1 - y2) * ( y1 - y2) )
  }
}

