import scala.io.StdIn
import scala.collection.mutable

object Main extends App {
  val n = StdIn.readLine().toInt
  var p : Array[Array[Int]] = Array.empty

  (0 to n - 1).foreach {i =>
    p = p :+ StdIn.readLine.split(" ").map(_.toInt)
  }

  

  val tmp = p.permutations
  var sum: Double = 0
  var cnt = 0

  tmp.foreach { j =>
    cnt = cnt + 1
    var prev :Array[Int] = Array.empty
    sum = sum + j.foldLeft[Double](0) {(acc, v) =>
      if (prev.isEmpty) {
        prev = v; 0
      } else {
        acc + Math.sqrt(Math.pow(prev(0) - v(0), 2) + Math.pow(prev(1) - v(1), 2))
      }
    }
  }
  print(sum / cnt)
}
