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: Accepted)

import java.util.ArrayDeque;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.HashMap;
import java.util.Map;
import java.util.Queue;
import java.util.Random;
import java.util.Scanner;
import java.util.Set;
 


public class Main {
	private static Scanner sc = new Scanner(System.in);
	//result = Math.pow(num1, num3)
	//StringBuffer str = new StringBuffer(hoge1);
	//String hoge2 = str.reverse().toString();
	//map.containsKey(A)

	//Map<Integer, Integer> map = new HashMap<Integer, Integer>(n);
	/*for ( キーのデータ型 key : マップの名前.keySet() ) {
		データのデータ型 data = マップの名前.get( key );
		
		// keyやdataを使った処理;
	}*/
	//int i = Integer.parseInt(s);
	//Queue<String> qq=new ArrayDeque<>(); //add,poll,peek
	//Deque<String> qq=new ArrayDeque<>();//push,pop,peek
	//ArrayList<Integer> cc = new ArrayList<>(N);
	//Collections.sort(list);
	//Array.sort(list);   cc.contains(tmp)
	//Arrays.asList(c).contains("a")
	//list.set(1,"walk");//1 1 2 3 5
	static long mod =1000000007;
	//static 	ArrayList<Integer> cc = new ArrayList<>(100);
	static int INF=1000000007;
	
	static int N,M,Ks[],Ps[];
	static boolean Ss[][];
	static int ctn=0;
	static int check(boolean v[]) {
		for(int i=0;i<M;i++) {
			int t=0;
			for(int a=0;a<N;a++) {
				if(Ss[i][a]&&v[a]) {
					t++;
				}
			}
			if(t%2!=Ps[i])return 0;
		}
		return 1;
	}
	public static void main(String[] args) {
		//int N=sc.nextInt();for(int i=0;i<N;i++) {}
		//int a[]=new int[N];for(int i=0;i<N;i++) {}
		//Ae2+Be2=Ce2
		//StringBuffer str = new StringBuffer(hoge1);
		//for(int i=0;i<N;i++) {}
		
		int H=sc.nextInt();
		int W=sc.nextInt();
		int h=sc.nextInt();
		int w=sc.nextInt();
		
		p(H*W+h*w-H*w-W*h);
	}
	
	static int dfs(int i,boolean v[]) {
		if(i==N) {
			ctn+=check(v.clone());
			return 1;
		}
		dfs(i+1,v.clone());
		v[i]=true;
		dfs(i+1,v.clone());
		
		return 1;
		
	}
	
	
		
	
	public static long modPow(long g,long N) {
		long T=1;
		for(int i=0;i<N;i++) {
			T*=g;
			T%=mod;
		}
		return T;
	}
	
	public static long lcm(long a, long b) {
        return a / gcd(a, b) * b;
	}
	
	public static long gcd(long a, long b) {
        return b == 0 ? a: gcd(b, a % b);
	}
	static String nextPermutation(String s) {
		ArrayList<Character> list=new ArrayList<>();
		for(int i=0;i<s.length();i++) {
			list.add(s.charAt(i));
		}
		int pivotPos=-1;
		char pivot=0;
		for(int i=list.size()-2;i>=0;i--) {
			if(list.get(i)<list.get(i+1)) {
				pivotPos=i;
				pivot=list.get(i);
				break;
			}
		}
		if(pivotPos==-1&&pivot==0) {
			return "Final";
		}
		int L=pivotPos+1,R=list.size()-1;
		
		
		
		int minPos=-1;
		char min =Character.MAX_VALUE;
		
		for(int i=R;i>=L;i--) {
			if(pivot<list.get(i)) {
				if(list.get(i)<min) {
					min=list.get(i);
					minPos=i;
				}
			}
		}
		
		Collections.swap(list, pivotPos, minPos);
		Collections.sort(list.subList(L, R+1));
		
		
		StringBuilder sb=new StringBuilder();
		for(int i=0;i<list.size();i++) {
			sb.append(list.get(i));
		}
		return sb.toString();
	}
	


	
	static void p(String ans) {System.out.println(ans);};
	static void p(int ans) {System.out.println(ans);};
	static void p() {System.out.println();};
	static void p(long ans) {System.out.println(ans);};
	static void p(double ans) {System.out.println(ans);};
	
}