import Control.Applicative
import Data.Function
import Control.Monad
import Data.List
import Data.Char
import Text.Printf

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

main :: IO ()
main = do
    a <- map read . words <$> getLine
    printf "%.10f\n" $ distance a
