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.io.IOException;
import java.io.InputStreamReader;
import java.io.BufferedReader;
import java.util.*;
import java.math.*;
import java.io.*;
import java.text.*;

class Main
{  

    
    public static boolean[] sieve(long n)
    {
        boolean[] prime = new boolean[(int)n+1];
        Arrays.fill(prime,true);
        prime[0] = false;
        prime[1] = false;
        long m = (long)Math.sqrt(n);
        for(int i=2;i<=m;i++)
        {
            if(prime[i])
            {
                for(int k=i*i;k<=n;k+=i)
                {
                    prime[k] = false;
                }
            }
        }
        return prime;
    } 

    
    static long GCD(long a,long b)
    {
        if(a==0 || b==0)
        {
            return 0;
        }
        if(a==b)
        {
            return a;
        }
        if(a>b)
        {
            return GCD(a-b,b);
        }
        return GCD(a,b-a);
    }

    static long CountCoPrimes(long n)
    {
        long res = n;
        for(int i=2;i*i<=n;i++)
        {
            if(n%i==0)
            {
                while(n%i==0)
                {
                    n/=i;
                }
                res-=res/i;
            }
        }
        if(n>1)
        {
            res-=res/n;
        }
        return res;
    }


    //fastest way to find x**n
    static long modularExponentiation(long x,long n,long m)
    {
        long res = 1;
        while(n>0)
        {
            if(n%2==1)
            {
                res = (res*x)%m;
            }
            x =(x*x)%m;
            n/=2;
        }
        return res;
    }

    static long lcm(long a,long b)
    {
        return (a*b)/GCD(a,b);
    }

   
    static long pow(long a,long b)
    {
        long res = 1;
        while(b>0)
        {
            if((b&1)==1)
            {
                res *= a;
            }

            b >>= 1;
            a *=a;
        }
        return res;
    }

   static long modInverse(long A,long M)
   {
    return modularExponentiation(A,M-2,M);
   }
 
   

    

    static boolean prime(int n)
    {
        for(int i=2;i*i<=n;i++)
        {
            if(i%2==0 ||i%3==0)
            {
                return false;
            }
        }
        return true;
    }

   


   
   
  
    public static void main(String[] args) throws IOException
    {
     
        new Main().run();
       
    }


    static int rev(int n)
    {
        int x=0;
        int num = 0;
        while(n>0)
        {
            x = n%10;
            num = num*10+x;
            n/=10;

        }
        return num;
    }

   
    static Scanner in = new Scanner(System.in);
    static void run() throws IOException
    {
        int W = ni();
        int H = ni();
        int w = ni();
        int h = ni();
        int rc = W*H;
        int tot= w*H;
        W=W-w;
        // System.out.println()
        tot += W*h;
        H-=h;
        System.out.println(rc-tot);
        

    }   


    

  
   
    

    
       
    

    
   
  



   

    static void printL(long a)
    {
        System.out.println(a);
    }
    static void printS(String s)
    {
        System.out.println(s);
    }

    static void printD(Double d)
    {
        System.out.println(d);
    }
  
   
   
    static void swap(char c,char p)
    {
        char t = c;
        c = p;
        p = t;
    }

    static long max(long n,long m)
    {
        return Math.max(n,m);
    }
    static long min(long n,long m)
    {
        return Math.min(n,m);
    }

    static double nd() throws IOException
    {
        return Double.parseDouble(in.next());
    }
    static int ni() throws IOException
    {
        return Integer.parseInt(in.next());
    }

    static long nl() throws IOException
    {
        return Long.parseLong(in.next());
    }

    static String si() throws IOException
    {
        return in.next();
    }


  
   
    static int abs(int n)
    {
        return Math.abs(n);
    }

    static class Scanner 
    {
        StringTokenizer st;
        BufferedReader br;

        public Scanner(InputStream s){  br = new BufferedReader(new InputStreamReader(s));}

        public String next() throws IOException 
        {
            while (st == null || !st.hasMoreTokens()) 
            {
                st = new StringTokenizer(br.readLine());
            }
            return st.nextToken();
        }

        public int nextInt() throws IOException {return Integer.parseInt(next());}
        
        public long nextLong() throws IOException {return Long.parseLong(next());}

        public String nextLine() throws IOException {return br.readLine();}

        public boolean ready() throws IOException {return br.ready();}


    }

    
}