import Control.Monad
import Text.Printf

comb 0 xs = [[]] 
comb _ [] = []
comb n (x:xs) = [x:y | y <- comb (n-1) xs] ++ comb n xs


calc ::  [[(Double, Double)]] -> Double -> Double 
calc [] sumv = sumv 
calc ((z1:z2:zs):xy) sumv = calc xy (sumv+len) 
  where
    v = sqrt $ ((fst z1) - (fst z2))^2 + ((snd z1) - (snd z2))^2
    len = v*2 


main = do
  n <- readLn 
  xy <- replicateM n $ do
    [a,b] <- map read . words <$> getLine :: IO [Double] 
    return (a,b)
 
  let xy_list = comb 2 xy
  let s = calc xy_list 0 
  let ans = s / (fromIntegral (n :: Int)) 
  printf "%.9f\n" ans
