import java.util.Scanner;

public class Main {

	@SuppressWarnings("resource")
	public static void main(String args[]) {
		Scanner scanner = new Scanner(System.in);
		int xa=scanner.nextInt();
		int ya=scanner.nextInt();
		int xb=scanner.nextInt();
		int yb=scanner.nextInt();
		xa*=-1;
		double a=(double)(yb-ya)/(xb-xa);
		double ans=ya-a*xa;
		System.out.println(ans);
	}
}