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 rec fact_rec x acc =
 match x with
 |0 -> acc
 |1 -> acc
 |_ -> fact_rec (x - 1) (x * acc)
let fact x = fact_rec x 1

let rec fact_s x y acc =
 match y with
 |0 -> acc
 |_ -> fact_s (x - 1) (y - 1) (x * acc)
let fact_ss x y = fact_s x y 1

let c k l = (fact_ss k l) / (fact l)

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

let k = (c n 2)
let cc = (c (k - 1) (n - 2))
let keisu = (float cc * 2.0) / (float (fact 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