import Numeric

showF :: RealFloat a => a -> String
showF = flip (showFFloat Nothing) ""

main :: IO ()
main = do
  [x1, y1, x2, y2] <- fmap (map read . words) getLine
  putStrLn . showF $ dist2d (x1, y1) (x2, y2)
    where dist2d (x1, y1) (x2, y2) =
            sqrt $ ((x1-x2)^2) + ((y1-y2)^2)