import Control.Monad
import Data.List

f ps = sum $ zipWith (\[x, y] [xx, yy] -> (sqrt $ (xx-x)**2 + (yy-y)**2)) ps $ tail ps

solve :: Int -> [[Double]] -> Double
solve n ps = flip (/) (fromIntegral $ product [1..n]) $ sum $ map f $ permutations ps

main = do
    n <- readLn
    ps <- replicateM n (map read . words <$> getLine)
    print $ solve n ps