import Control.Applicative
import Data.List

main = do
    xs <- init . map (map (read :: String -> Double) . words) . lines <$> getContents
    let x = do
        [r0, w0, c, r] <- xs
        return . show . max 0 . ceiling $ (w0*c-r0) / r
    putStrLn . intercalate "\n" $ x
