import Control.Applicative
    
type Point = (Double, Double)

type Problem = (Point, Point)

type Result = Double
        
solve :: Problem -> Result

solve ((x1, y1), (x2, y2))
    = sqrt $ dx ** 2 + dy ** 2 where
        dx = x1 - x2
        dy = y1 - y2

output :: Result -> String

output result
    = show result
    
input :: IO Problem

input
    = do
        [x1, y1, x2, y2] <- map read.words <$> getLine
        return ((x1, y1), (x2, y2))

main :: IO ()

main
    = putStrLn.output =<< solve <$> input