import Control.Monad
import Data.List

main = do
  n <- readLn
  xys <- replicateM n $ (map read . words <$> getLine) :: IO [[Double]]
  print $ solve xys

solve xys =
  let xyss = permutations xys
  in
  	(/(fromIntegral $ length xyss)) . sum . map f $ xyss
  where
    d [x1, y1] [x2, y2] = sqrt $ (x1 - x2)**2 + (y1 - y2)**2
    
    f [x, y] = d x y
    f (x:y:xys) = d x y + f (y:xys)
