import java.util.Scanner;
public class Main{
public static void main(String[] args)
{
Scanner sc = new Scanner(System.in);
int a = sc.nextInt();
int b = sc.nextInt();
int c = sc.nextInt();
int d = sc.nextInt();
System.out.printf("DIFERENCA = %d\n", a*b-c*d);
}
}