import Data.Functor

default (Integer, Double)

main :: IO ()
main = do
    point_list <- map (\x  -> read x::Double) <$> words <$> getLine
    print (distance point_list)

distance::[Double] -> Double
distance ps =
        sqrt ((ps!!0 - ps!!2)^2 + (ps!!1 - ps!!3)^2)