import Control.Applicative ((<$>), (<*>))

main :: IO ()
main = do
  solve <$> getl (map read . words) <*> getl (map read . words) >>= print

solve :: [Int] -> [Int] -> Double
solve [px, py] [qx, qy] = fromIntegral (abs (qx - px) + abs (qy - py)) / 2.0

getl :: (String -> a) -> IO a
getl f = f <$> getLine