import scala.collection.mutable._
import scala.math._
object Main {
  def distance(pt1: (Int, Int), pt2: (Int, Int)): Double = {
     val (x1, y1) = pt1
     val (x2, y2) = pt2
     sqrt((x1 - x2) * (x1 - x2) + (y1 - y2) * (y1 - y2))
  }
  def main(args: Array[String]): Unit = {
    val N = Console.in.readLine().toInt
    val pts = Buffer.empty[(Int, Int)]
    for(line <- Iterator.continually(Console.in.readLine()).takeWhile(_ != null).toList) {
      val Array(x, y, _*) = line.split(" ")
      pts += ((x.toInt, y.toInt))
    }
    val routes = pts.toList.permutations.toList
    var all = 0.0
    for(route <- routes) {
      all += (route zip route.tail).map{ case (p1, p2) => distance(p1, p2) }.sum
    }
    println(all / routes.length)
  }
}