import java.util.*;

public class Main {
	public static void main (String[] args) {
		Scanner sc = new Scanner(System.in);
		int xa = sc.nextInt();
		int ya = sc.nextInt();
		int xb = sc.nextInt();
		int yb = sc.nextInt();
		double left = Math.min(ya, yb);
		double right = Math.max(ya, yb);
		for (int i = 0; i < 1000000; i++) {
		    double m1 = (left * 2 + right) / 3;
		    double m2 = (left + right * 2) / 3;
		    if (getDist(xa, ya, xb, yb, m1) <= getDist(xa, ya, xb, yb, m2)) {
		        right = m2;
		    } else {
		        left = m1;
		    }
		}
		System.out.println(right);
	}
	
	static double getDist(int x1, int y1, int x2, int y2, double yy) {
	    return Math.sqrt(x1 * x1 + (y1 - yy) * (y1 - yy)) + Math.sqrt(x2 * x2 + (y2 - yy) * (y2 - yy));
	}
}