{-# LANGUAGE BangPatterns #-}
{-# LANGUAGE BinaryLiterals #-}
{-# LANGUAGE MultiWayIf #-}
{-# LANGUAGE NumDecimals #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TupleSections #-}

import Control.Applicative
import Control.Arrow
import Control.Monad
import Control.Monad.ST

import Data.Char
import Data.List
import Data.Maybe

import qualified Data.ByteString.Char8 as B

import Data.Array.ST.Safe
import Data.STRef

import Debug.Trace
import Text.Printf

readInt = readLn :: IO Int
readInteger = readLn :: IO Integer
readInts = map ( fst . fromJust . B.readInt ) . B.words <$> B.getLine
readIntegers = map ( fst . fromJust . B.readInteger ) . B.words <$> B.getLine

which a b f = if f then a else b
mp [ a, b ] = ( a, b )

modifyArray a i f = writeArray a i =<< f <$> readArray a i

main = do
	n <- readInt
	[ xs, ys ] <- map ( map fromIntegral ) . transpose <$> replicateM n readInts
	let
		ps = permutations [ 0 .. n - 1 ]
		m = fromIntegral $ length ps
		s = sum $ map ( solve xs ys ) ps
	printf "%12f\n" $ ( s / m :: Double )

solve _ _ [_] = 0
solve xs ys (i:j:ps) = d + solve xs ys (j:ps)
	where
		x1 = xs !! i
		y1 = ys !! i
		x2 = xs !! j
		y2 = ys !! j
		d = sqrt $ ( x1 - x2 ) ** 2 + ( y1 - y2 ) ** 2
