import Control.Monad
import Data.List
main = do
  n <- readLn
  [xs, ys] <- transpose . map (map read . words) <$> replicateM n getLine
  let
    a = sum $ map (solve xs ys) $ permutations [0..n-1]
    b = fromIntegral $ product [1..n]
  print (a / b :: Double)

solve :: [Double] -> [Double] -> [Int] -> Double
solve _ _ [_] = 0
solve xs ys (i:j:ps) = sqrt ((xs!!i - xs!!j)^2 + (ys!!i - ys!!j)^2) + solve xs ys (j:ps)
