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 Java8 (OpenJDK 1.8.0) (judge verdict: Runtime Error)

import java.util.ArrayList;
import java.util.Collections;
import java.util.Scanner;
public class Main {
  static class AAA implements Comparable<AAA> { // ★ @Override する場合， implements Comparable<AAA> が必要
    int price;
    int number;

    public AAA(int a, int b) {
      price = a;
      number = b;
    }

    @Override
    public int compareTo(AAA a) {
      return this.price - a.price;
    }
  }

  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    // スペース区切りの整数の入力(NとM)
    int N = sc.nextInt();
    int M = sc.nextInt();
    long total = 0;
    int[] A = new int[N];
    int[] B = new int[N];
    for (int k = 0; k < N; k++) {
      A[k] = sc.nextInt();
      B[k] = sc.nextInt();
    }
    ArrayList<AAA> shopInfo = new ArrayList<>();
    for (int k = 0; k < N; k++) {
      shopInfo.add(new AAA(A[k], B[k]));
    }
    Collections.sort(shopInfo);
    for (int k = 0; k < N; k++) {
      if (shopInfo.get(k).number >= M) { // ★ [k] ではなく，.get(k)
        total = total + shopInfo.get(k).price * M;// ★ [k] ではなく，.get(k)
        break;
      } else {
        total = shopInfo.get(k).price * shopInfo.get(k).number; // ★ [k] ではなく，.get(k)
        M = M - shopInfo.get(k).number; // ★ [k] ではなく，.get(k)
      }
    }
    System.out.println(total);
  }
}