import java.util.Scanner;

public class Main {
	static double xa;
	static double ya;
	static double xb;
	static double yb;
	public static void main(String[] args) {
		Scanner scan = new Scanner(System.in);
		xa = scan.nextDouble();
		ya = scan.nextDouble();
		xb = scan.nextDouble();
		yb = scan.nextDouble();
		scan.close();
		double yp = 9;
		double r = Math.max(ya, yb);
		double l = Math.min(ya, yb);
		while(Math.abs(r - l) > 0.0000000001) {
			double y1 = (2.0 * l + r) / 3.0;
			double y2 = (l + 2.0 * r) / 3.0;
//			System.out.println(l + " " + r);
//			System.out.println(y1 + " " + y2);
//			System.out.println(diff1_func(y1) + " " + diff1_func(y2));
//			System.out.println();
//			if(Math.abs(diff1_func(y1)) < Math.abs(diff1_func(y2))) {
//				r = y2;
//			}else {
//				l = y1;
//			}
			if(func(y1) < func(y2)) {
				r = y2;
			}else {
				l = y1;
			}
		}

		yp = (l + r) / 2.0;
		System.out.println(yp);
		System.exit(0);
		for(int i = 0; i < 100; i++) {
			double yp_new = yp - diff1_func(yp) / diff2_func(yp);
			yp = yp_new;
			System.out.println(yp);
		}
		System.out.println(yp);
	}
	static double func(double yp) {
		return Math.pow(func1(yp), 0.5) +  Math.pow(func2(yp), 0.5);
	}
	static double diff1_func(double yp) {
		return (yp - ya) * Math.pow(xa * xa + (yp - ya) * (yp - ya), -0.5)
				+ (yp - yb) * Math.pow(xb * xb + (yp - yb) * (yp - yb), -0.5);
	}
	static double diff2_func(double yp) {
		return Math.pow(func1(yp), -0.5) - Math.pow(yp - ya, 2) * Math.pow(func1(yp), -1.5) +
				Math.pow(func2(yp), -0.5) - Math.pow(yp - yb, 2)* Math.pow(func2(yp), -1.5);
	}
	static double func1(double yp) {
		return xa * xa + (yp - ya) * (yp - ya);
	}
	static double func2(double yp) {
		return xb * xb + (yp - yb) * (yp - yb);
	}
}