Problem:
Score : 100 points

Problem Statement

There are H rows and W columns of white square cells.

You will choose h of the rows and w of the columns, and paint all of the cells contained in those rows or columns.

How many white cells will remain?

It can be proved that this count does not depend on what rows and columns are chosen.

Constraints

All values in input are integers.

1 \leq H, W \leq 20

1 \leq h \leq H

1 \leq w \leq W

Input

Input is given from Standard Input in the following format:

H W
h w

Output

Print the number of white cells that will remain.

Sample Input 1

3 2
2 1

Sample Output 1

1

There are 3 rows and 2 columns of cells. When two rows and one column are chosen and painted in black, there is always one white cell that remains.

Sample Input 2

5 5
2 3

Sample Output 2

6

Sample Input 3

2 4
2 4

Sample Output 3

0

Solution in Haskell (GHC 7.10.3) (judge verdict: Accepted)

{-# LANGUAGE CPP #-}
module Main where

import qualified Data.Vector as V 
import qualified Data.Map.Strict as M
import Control.Monad
import qualified Data.ByteString.Char8 as BS
import Data.Maybe

dump :: (Show a) => a -> IO()
#if __GLASGOW_HASKELL__ >= 850
dump x = print x
#else
dump _ = pure ()
#endif

readint :: IO Int
readint = (fst . fromJust . BS.readInt) <$> BS.getLine

readvec :: IO [Int]
readvec = (fmap (read . BS.unpack) . BS.words) <$> BS.getLine

newtype UF a = UF (M.Map a a) deriving (Show) -- Map node parent

makeuf :: Int -> UF Int
makeuf n = let m = M.fromList $ zip [0..n] [0..n]
                in UF m

union :: (Eq a, Ord a) => a -> a -> UF a -> UF a
union l r uf@(UF m) = let lp = find l uf
                          rp = find r uf
                      in if lp == rp
                         then uf
                         else if size l uf < size r uf
                              then UF $ M.update (const $ Just rp) lp m
                              else UF $ M.update (const $ Just lp) rp m

find :: (Eq a, Ord a) => a -> UF a -> a
find x uf@(UF m) = let px = m M.! x
                in if px == x
                   then px
                   else find px uf -- TODO 貼り直し

size :: (Eq a, Ord a) => a -> UF a -> Int
size x (UF m) = M.size $ M.filter (\par -> par==x) m

main :: IO()
main = do
  [h, w] <- readvec
  [h', w'] <- readvec
  print $ (h - h') * (w - w')