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: Wrong Answer)

module Main (main) where

import Control.Monad
import Control.Monad.Trans.State
import qualified Data.ByteString.Unsafe as B
import qualified Data.ByteString.Char8 as C8
import Data.Bool
import Data.Char
import Data.List
import Data.Maybe

main :: IO ()
main =
  replicateM 2 readPair >>= print . showAnswer . solve

type Input  = [(Int, Int)]
type Output = Int

solve :: Input -> Output
solve [(h1,w1),(h2,w2)] = (h1-h2) * (h2-w2)

showAnswer :: Output -> Answer
showAnswer = Number

-- utils
readInts :: IO [Int]
readInts = map (fst . fromJust . C8.readInt) . C8.words <$> C8.getLine

readPair :: IO (Int, Int)
readPair = head . unfoldr readT <$> C8.getLine
  where
    readT = runStateT $
      (,) <$> StateT (C8.readInt . C8.dropWhile isSpace)
          <*> StateT (C8.readInt . B.unsafeTail)
          
data Answer
  = YesNo Bool
  | YESNO Bool
  | AB Bool
  | Number Int
  | String String
  | Compare Ordering
  | Compare2 Ordering

instance Show Answer where
  show (YesNo b)    = bool "No" "Yes" b
  show (YESNO b)    = bool "NO" "YES" b
  show (AB b)       = bool "B"  "A" b
  show (Number i)   = show i
  show (String s)   = s
  show (Compare o)  = ordering "<" "=" ">" o
  show (Compare2 o) = ordering "Right" "Balanced" "Left" o

ordering :: a -> a -> a -> Ordering -> a
ordering lt eq gt o = case o of
  LT -> lt
  EQ -> eq
  GT -> gt