import Control.Applicative
import Control.Monad
import qualified Data.ByteString.Char8 as B
import Data.Maybe (fromJust)
import Text.Printf
import Debug.Trace

readInts :: B.ByteString -> [Int]
readInts = map (fst . fromJust . B.readInt) . B.words

getInts :: IO [Int]
getInts = liftM readInts B.getLine

main = do
  [a,b] <- getInts
  [c,d] <- getInts
  print $ (fromIntegral (abs (a-c)) / 2.0) + (fromIntegral (abs (b-d)) / 2.0)