let fs, s, n = Array.make 8 0, ref 0., Scanf.scanf " %d" (+) 0
let xys = Array.init n @@ fun _ -> Scanf.scanf " %d %d" @@ fun a b -> a, b
let g (a, b) (c, d) = sqrt @@ float @@ (a - c) * (a - c) + (b - d) * (b - d)
let rec h n = if n < 1 then 1 else n * h (n - 1)
let rec f i a p = if i >= n then s := !s +. a else (for j = 0 to n - 1 do
  if fs.(j) = 0 then (fs.(j) <- 1; f (i + 1) (if p < 0 then a else a +. g xys.(j) xys.(p)) j; fs.(j) <- 0) done)
let _ = f 0 0. ~-1; Printf.printf "%.10f\n" @@ !s /. float (h n)