import io.StdIn.{readInt, readLine}
import math.{sqrt, pow}
object Main extends App {
  val n = readInt
  val xys = for (_ <- 1 to n) yield readLine.split(" ").map(_.toInt)

  def dis(p1: Array[Int], p2: Array[Int]) =
    sqrt(pow(p1.head-p2.head, 2)+pow(p1.last-p2.last, 2))

  val distances = for (route <- xys.permutations; edge <- route.sliding(2))
    yield dis(edge.head, edge.last)

  val result = (distances.sum) / ((1 to n).product)
  println(result)
}