import java.io.{BufferedReader, InputStreamReader}

object Main {
  def main(args: Array[String]): Unit = {
    val in = new BufferedReader(new InputStreamReader(System.in))
    val n = in.readLine().toInt
    val nodes = (0 until n).map { _ =>
      in.readLine().split("\\s+").map(_.toInt)
    }.toArray

    val lengths = for {
      n1 <- nodes.indices
      n2 <- nodes.indices
      if n1 < n2
    } yield {
      val node1 = nodes(n1)
      val node2 = nodes(n2)

      Math.sqrt(Math.pow(node2(0) - node1(0), 2.0) + Math.pow(node2(1) - node1(1), 2.0))
    }


    System.err.println(
      lengths
    )

//    val result = lengths.sum * factorial(n-2) * (n-1) * 2 / factorial(n)
    val result = lengths.sum / n * 2
    System.out.println(
      f"$result%2.10f"
    )
  }

  def factorial(n: Int): Long = {
    def inner(n: Int, cur: Long): Long = {
      if (n == 0) {
        cur
      } else {
        inner(n - 1, cur * n)
      }
    }

    inner(n, 1)
  }
}
