import java.util.Scanner;

fun main (args : Array<String>) {
  val input = Scanner(System.`in`)
  
  while (input.hasNext()) {
    val N = input.nextInt()
    val x = DoubleArray(N)
    val y = DoubleArray(N)
    
    for (i in x.indices) {
      x[i] = input.nextDouble()
      y[i] = input.nextDouble()
    }
    
    var pathLen = 0.0
    for (i in x.indices) {
      for (j in (i + 1) .. x.lastIndex) {
        val xDiff = x[i] - x[j]
        val yDiff = y[i] - y[j]
        
        pathLen += Math.sqrt(xDiff * xDiff + yDiff * yDiff) * 2
      }
    }
    
    println(pathLen / N)
  }
}