import Control.Monad
import Control.Applicative
import Data.List

main :: IO()
main = getLine >>= main' . map read . words
    where 
        main' [0, 0, 0, 0] = return ()
        main' as = do
            print $ solve as 0
            main

solve :: [Int] -> Int -> Int
solve [r0, w0, c, r] n = if div r0 w0 >= c
                             then n
                             else solve [r0+r, w0, c, r] n+1