import scala.io.StdIn._
object Main extends App {
  val n = readInt
  val cities = (1 to n).map{i => readLine.split(" ").map(_.toInt)}
  def distance(i: Array[Int], j: Array[Int]): Double = 
    Math.sqrt((i(0) - j(0)) * (i(0) - j(0)) + (i(1) - j(1)) * (i(1) - j(1)))
  val distances = (for {
    i <- 0 until n - 1
    j <- i until n
  } yield distance(cities(i), cities(j))).sum
  println(2 * distances / n)
}