let n = stdin.ReadLine() |> int
let r = [1..n] |> List.map (fun x -> stdin.ReadLine().Split() |> Array.map double |> fun x -> (x.[0],x.[1]))
let d (x1,y1) (x2,y2) = ((x1-x2)**2.0 + (y1-y2)**2.0)**(0.5)

let rec f i s =
    if i=n then s*2.0/(double n)
    else
        let sl = [(i+1)..(n-1)] |> List.map (fun x -> d (r.[i]) (r.[x])) |> List.sum
        f (i+1) (s + sl)

f 0 0.0 |> stdout.WriteLine