import java.util.Scanner
import java.io.PrintWriter

object Main {
  def main(args: Array[String]): Unit = {
    val sc = new Scanner(System.in)
    val pw = new PrintWriter(System.out)

    val n = sc.nextInt()
    val tuples: List[(Double, Double)] = List.fill(n)((sc.nextInt(), sc.nextInt()))
    var sum: Double = 0
    for (i <- 0 until n) {
      for (j <- i + 1 until n) {
        sum += math.sqrt((tuples(i)._1 - tuples(j)._1) * (tuples(i)._1 - tuples(j)._1)
                      + (tuples(i)._2 - tuples(j)._2) * (tuples(i)._2 - tuples(j)._2))
      }
    }
    pw.println(sum * 2 / n)


    pw.flush()
  }
}