import Control.Applicative
import Text.Printf

distance :: Floating a => [a] -> a
distance [x1, y1, x2, y2] = sqrt $ (x2 - x1) ^ 2 + (y2 - y1) ^ 2

main :: IO ()
main = printf "%.8f\n" =<< (distance . map (read :: String -> Double) . words) <$> getLine