import scala.io.StdIn._
object Main extends App {
  val n = readInt
  val xy = (0 until n).map { _ =>
    readLine.split(" ").map(_.toInt) match {
      case Array(x, y) => (x, y)
    }
  }

  val p = (0 until n).permutations
  var result = 0d
  var count = 0
  for (l <- p) {
    count = count + 1
    var prev = xy(l.head)
    for (i <- l.tail) {
      val d = math.sqrt(
        math.pow(prev._1 - xy(i)._1, 2) + math.pow(prev._2 - xy(i)._2, 2)
      )
      result = result + d
      prev = xy(i)
    }
  }
  println(result / count)

}
