import Control.Applicative ((<$>))
import Data.Int (Int64)

judge :: Integral a => a -> a -> a -> a -> Bool
judge h w n k = (h + w - 2) `mod` n == k - 1

main :: IO ()
main = do
    [h, w, n, k] <- fmap read . words <$> getLine :: IO [Int64]
    putStrLn $ if judge h w n k then "YES" else "NO"