import scala.math.{pow, sqrt}

object Main extends App {
  
  val sc = new java.util.Scanner(System.in)
  val n = sc.nextInt()
  val c = Array.fill(n)(sc.nextLong(), sc.nextLong())
  val total = c.permutations.map(path => {
    path.sliding(2).foldLeft(0d)((sum, route) => sum + dist(route))
  }).toList
  println(total.sum / total.length)
  
  def dist(x: Array[(Long, Long)]): Double = {
    sqrt(pow((x(0)._1 - x(1)._1), 2) + pow((x(0)._2 - x(1)._2), 2))
  }
}
