main = do
    fst <- getLine
    snd <- getLine
    let h = head $ intList fst
    let w = head $ reverse $ intList fst
    let a = head $ intList snd
    let b = head $ reverse $ intList snd
    let y = show $ ans h w a b
    putStrLn y

intList :: [Char] -> [Int]
intList xs = map read (words xs) 

ans :: Int -> Int -> Int -> Int -> Int
ans h w a b = (h - a) * (w - b)
