import Text.Printf

main = do
    ss <- getLine
    let ns = map (\x -> read x :: Double) $ words ss
    let (x1:y1:x2:y2:_) = ns
    let ans = dist x1 y1 x2 y2
    printf "%.8f\n" ans
    
dist :: Double -> Double -> Double -> Double -> Double
dist x1 y1 x2 y2 = sqrt $ (x1 - x2)^2 + (y1 - y2)^2
