[<AutoOpen>]
module Program

open System

let read f = stdin.ReadLine() |> f
let reada f = stdin.ReadLine().Split() |> Array.map f
let reads f = stdin.ReadLine().Split() |> Array.toList |> List.map f

let dist [x1;y1] [x2;y2] =
    abs (x1 - x2) ** 2.0 + abs (y1 - y2) ** 2.0
    |> sqrt

let distAll ps =
    List.pairwise ps
    |> List.fold (fun s (p1, p2) -> s + dist p1 p2) 0.0

let rec insertions x = function
        | []             -> [[x]]
        | (y :: ys) as l -> (x::l)::(List.map (fun x -> y::x) (insertions x ys))

let rec permutations = function
    | []      -> seq [ [] ]
    | x :: xs -> Seq.concat (Seq.map (insertions x) (permutations xs))

[<EntryPoint>]
let main _ =
    let N = read int
    let XYS = List.init N (fun _ -> reads float)

    let pss = permutations XYS |> List.ofSeq

    let ans =
        List.fold (fun s ps -> s + distAll ps) 0.0 pss

    let k = List.fold ( * ) 1 [1 .. N]

    // eprintfn "*%A" pss
    // eprintfn "*%A" ans
    // eprintfn "*%A" k

    printfn "%.7f" (ans / float k)
    0