import Data.List

main :: IO ()
main = do
  l1 <- getLine
  let n = read l1 :: Int
  ln <- sequence (replicate n getLine)
  let xys = map (map read . words) ln :: [[Double]]
      (xs, ys) = (map head xys, map (head . tail) xys)
      xys' = zip xs ys
      routes = permutations [0..n-1]
  print $ sum (map (\r -> sum $ zipWith (\i1 i2 -> f (xys' !! i1) (xys' !! i2)) r $ tail r) routes)/fromIntegral (length routes)

f (x1, y1) (x2, y2) = sqrt $ (x2 - x1)**2 + (y2 - y1)**2