open System
let n = stdin.ReadLine() |> int
let xy = Array.init n (fun _ -> stdin.ReadLine().Split() |> Array.map int |> (fun [|a;b|] -> (a,b)))

let d (x1,y1) (x2,y2) = Math.Sqrt (((float x1) - (float x2))**2.0 + ((float y1) - (float y2))**2.0)

let keisu = 2.0 / (float n)

let D = [for i in 0..(n - 1) do
         for j in (i + 1)..(n - 1) do
         yield (d xy.[i] xy.[j])] |> List.sum

D * keisu
|> stdout.WriteLine