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 Ruby (2.3.3) (judge verdict: Accepted)

# H 行 W 列の白色のマス目があります。
# あなたは h 個の行と w 個の列を選び、
# 選んだ行または列に含まれるマス目を全て黒色で塗りつぶします。
# 白色のマス目はいくつ残るでしょうか。
# なお、残る白色のマス目の数は行や列の選び方によらないことが証明できます。

input_line_1 = gets.chomp.split
H = input_line_1[0].to_i
W = input_line_1[1].to_i

input_line_2 = gets.chomp.split
h = input_line_2[0].to_i
w = input_line_2[1].to_i

puts (H * W) - ((h * W) + (w * (H - h)))