import Control.Applicative ((<$>), (<*>))

main :: IO ()
main = solve <$> (map read <$> words <$> getLine) <*> (map read <$> words <$> getLine) >>= print

solve :: [Int] -> [Int] -> Double
solve [xa, ya] [xb, yb] = fromIntegral ya + (fromIntegral xa) * (fromIntegral yb - fromIntegral ya) / (fromIntegral xa + fromIntegral xb)