import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        double x1 = sc.nextInt();
        double y1 = sc.nextInt();
        double x2 = -sc.nextInt();
        double y2 = sc.nextInt();
        double ans = y1 - x1 * (y1 - y2) / (x1 - x2);
        System.out.println(ans);
    }
}