import Control.Monad
import Data.Maybe
import qualified Data.ByteString.Char8 as BS
import Data.List

readInt = fst . fromJust . BS.readInt
readIntList = map readInt . BS.words
getInt = readInt <$> BS.getLine
getIntList = readIntList <$> BS.getLine

main = do
    n <- getInt
    xys <- replicateM n getIntList
    print $ sum [dist xi yi xj yj | [xi, yi] <- xys, [xj, yj] <- xys] / (fromIntegral n)

dist xi yi xj yj = sqrt . fromIntegral $ (xi - xj) ^ 2 + (yi - yj) ^ 2