import Control.Monad

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

solve :: Int -> [[Double]] -> Double
solve n xys = sum [ sqrt ((x1 - x2) ^ 2 + (y1 - y2) ^ 2) | [x1, y1] <- xys, [x2, y2] <- xys ]  / (fromIntegral n)