import Control.Applicative
import Text.Printf

solve :: [Double] -> Double
solve [x1, y1, x2, y2] = sqrt $ x * x + y * y
    where
        x = x1 - x2
        y = y1 - y2

main :: IO ()
main = getLine >>= putStrLn <$> printf "%.8f" <$> solve <$> map read <$> words