import Control.Applicative ((<$>))
import Text.Printf (printf)

type Point a = (a, a)

manhattanDist :: Integral a => Point a -> Point a -> a
manhattanDist (x1, y1) (x2, y2) = abs (x2 - x1) + abs (y2 - y1)

minTime :: (Integral a, RealFloat b) => Point a -> Point a -> b
minTime p1 p2 = fromIntegral (manhattanDist p1 p2) / 2.0

tuplify2 :: [a] -> (a, a)
tuplify2 [x, y] = (x, y)
tuplify2 _ = error "Length of the list must be two"

main :: IO ()
main = do
    p1 <- tuplify2 . fmap read . words <$> getLine :: IO (Point Int)
    p2 <- tuplify2 . fmap read . words <$> getLine :: IO (Point Int)
    printf "%.10f\n" (minTime p1 p2 :: Double)