import java.util.Scanner;
public class Main {
public static void main
(String[] args
) { Scanner scanner
= new Scanner
(System.
in);
long A = scanner.nextInt();
long B = scanner.nextInt();
long C = scanner.nextInt();
long D = scanner.nextInt();
long Difference = (A*B)-(C*D);
System.
out.
print("Difference = "+Difference
); scanner.close();
}
}
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