import Control.Monad
import Data.List

main = do
  n <- readLn
  ps <- map (map read . words) <$> replicateM n getLine
  print $ solve ps

solve :: [[Double]] -> Double
solve xss = sum rs / (fromIntegral . length) rs
  where
    rs = map calcDistance . permutations $ xss

calcDistance :: [[Double]] -> Double
calcDistance xs  = snd $ foldr helper (head xs, 0) (tail xs)
  where
    helper [px1, py1] ([px0,py0], acc) = ([px1,py1], acc + sqrt ((px1-px0) ** 2 + (py1-py0) ** 2))