Untitled

import java.util.Scanner;

public class Main
{
	public static void main(String[] args) {
	    Scanner scanner = new Scanner(System.in);
		double a11 = scanner.nextDouble();
		double a12 = scanner.nextDouble();
		double a21 = scanner.nextDouble();
		double a22 = scanner.nextDouble();
		
		System.out.printf("%.4f", a11 * a22 - a12 * a21);
	}
}