import java.util.*

fun main(args: Array<String>) {
  val sc = Scanner(System.`in`)
  val n = sc.nextInt()
  val xy = (1..n).map { Pair(sc.nextInt(), sc.nextInt()) }

  val dlist = mutableListOf<Int>()
  for (i in 0 until n) {
    for (j in i + 1 until n) {
      val x = xy[i].first - xy[j].first
      val y = xy[i].second - xy[j].second
      val tmp = x * x + y * y
      dlist.add(tmp)
    }
  }

  val dsum = dlist.map { Math.sqrt(it.toDouble()) }.sum()

  println(dsum * 2.0 / n.toDouble())
}
