Public Class Program
  Shared Sub Main()
    Dim n As Double = Double.Parse(Console.ReadLine())
    Dim xs(0 To n - 1) As Double
    Dim ys(0 To n - 1) As Double
    For i As Double = 0 To n - 1
      Dim xy As Double() = Array.ConvertAll(Of String, Double)(Console.ReadLine().Split(" "), AddressOf Double.Parse)
      xs(i) = xy(0)
      ys(i) = xy(1)
    Next
    Dim l As Double = 0 ' 町iと町jの間の距離の総和 '
    For i As Double = 0 To n - 2
      For j As Double = i + 1 To n - 1
        l += Math.Sqrt(((xs(i) - xs(j)) * (xs(i) - xs(j))) + ((ys(i) - ys(j)) * (ys(i) - ys(j))))
      Next
    Next
    ' N!通りの経路のうち、各町のペア間の移動が発生するような経路は 2(N-1)!通りある '
    ' よって、N!通りの経路の長さの平均値は l × 2(N-1)! ÷ N! = l × 2 ÷ N '
    Dim ans As Double = l * 2 / n
    Console.WriteLine(ans.ToString())
  End Sub
End Class
