import           Control.Applicative ((<$>))

findP :: Double -> Double -> Double -> Double -> Double
findP xa ya xb yb = ya - t * xa
    where t = (ya - yb) / (xa + xb)

main :: IO ()
main = do
    [xa, ya] <- map read . words <$> getLine
    [xb, yb] <- map read . words <$> getLine
    print $ findP xa ya xb yb