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 C# (Mono 4.6.2.0) (judge verdict: Accepted)

using System;
using System.Linq;
using System.Collections.Generic;
using System.Collections;
public class Program {
  public static void Main() {
    int[] HW = ReadIntArray();
    int[] hw = ReadIntArray();
    int a = hw[0] * HW[1] + hw[1] * HW[0];
    int b = a - hw[0]*hw[1];
    int res = HW[0] * HW[1] - b;
    Console.WriteLine(res);
  }

  private static string ReadStr() { return Console.ReadLine(); }
  private static int ReadInt() { return int.Parse(Console.ReadLine()); }
  private static double ReadDouble() { return double.Parse(Console.ReadLine()); }
  private static long ReadLong() { return long.Parse(Console.ReadLine()); }
  private static char ReadChar() { return Console.ReadLine()[0]; }
  private static string[] ReadStringArray() { return Console.ReadLine().Split(' '); }
  private static int[] ReadIntArray() { return Console.ReadLine().Split(' ').Select(int.Parse).ToArray(); }
  private static double[] ReadDoubleArray() { return Console.ReadLine().Split(' ').Select(double.Parse).ToArray(); }
  private static long[] ReadLongArray() { return Console.ReadLine().Split(' ').Select(long.Parse).ToArray(); }
  private static char[] ReadCharArray() { return Console.ReadLine().ToArray(); }
  private static string[] ReadCharStringArray() { return Console.ReadLine().ToCharArray().Select(c => c.ToString()).ToArray(); }
}