object Main extends App {
  val sc = new java.util.Scanner(System.in)
  val N = sc.nextInt()
  val lengths =
    List.fill(N)(sc.nextInt() -> sc.nextInt).combinations(2).map {
      case Seq((x1, y1), (x2, y2)) =>
        math.sqrt(math.pow(x1 - x2, 2) + math.pow(y1 - y2, 2))
    }
  println(lengths.sum * 2 / N)
}
