module Main where
--import Debug.Trace
import Data.List
import Data.Char
import Data.Array
import Data.Maybe
import Data.Ord
import Control.Monad

solve :: [Double] -> Double
solve [ax,ay,bx,by] = sqrt $ (ax-bx)^2+(ay-by)^2

readLine:: IO [Double]
readLine = fmap (map read . words) getLine

main :: IO() 
main = do
    l <- readLine
    print (solve l)

