import Text.Printf

data Point = Point Double Double deriving Show
dist (Point x1 y1) (Point x2 y2) = sqrt $ (x1 - x2) * (x1 - x2) + (y1 - y2) * (y1 - y2) 

main = do
  [x1, y1, x2, y2] <- return . map (\x -> read x :: Double) . words =<< getLine
  let p1 = Point x1 y1
  let p2 = Point x2 y2
  putStrLn $ printf "%.08f" $ dist p1 p2