import Control.Monad
import Data.List
import Text.Printf
main=do
  n <- readLn
  [xs, ys] <- transpose <$> map (map (read::String->Double) . words) <$> replicateM n getLine
  let
    ps = permutations [0..n-1]
    a = sum $ map (solve xs ys) ps
    b = fromIntegral $ product [1..n]
  printf "%10f" (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)