{-# LANGUAGE FlexibleContexts, BangPatterns #-}

module Main where

import Data.Char
import Data.List
import Control.Monad
import Control.Monad.ST
import Data.Array.ST
import Data.Array.Unboxed
import Control.Applicative
import Text.Printf


readInt :: IO Double
readInt = read <$> (input `seq` input)
    where input = do
            !c <- getChar
            if isDigit c || c == '-'
                then (c :) <$> input
                else return []

prints :: Show a => [a] -> IO ()
prints as = putStrLn $ concat $ intersperse " " $ map show as

main = putStrLn . printf "%.7f" . f =<< replicateM 4 readInt

f [x1,y1,x2,y2] = sqrt $ (x1 - x2) ^ 2 + (y1 - y2) ^ 2
