import Control.Applicative ((<$>))

detClass :: Integral a => a -> a -> a
detClass m x
    | d <= m - 1 = d
    | otherwise = 2 * m - d - 1
    where d = mod (x - 1) $ 2 * m

main :: IO ()
main = do
    [_, m] <- fmap read . words <$> getLine :: IO [Int]
    [x, y] <- fmap read . words <$> getLine :: IO [Int]
    putStrLn $ if detClass m x == detClass m y then "YES" else "NO"