import Control.Monad

main :: IO ()
main = do
  n <- readLn
  let n' = fromIntegral n
  xys <- map (map (read :: String -> Double) . words) <$> getNLine_ n
  print $ ans n' xys

ans :: Double -> [[Double]] -> Double
ans n' xys = (/n') . sum $ distance <$> xys <*> xys where
  distance [a,b] [c,d] = sqrt $ (a-c)^2 + (b-d)^2

getNLine_ :: Int -> IO [String]
getNLine_ n = replicateM n getLine
