import java.util.*;
import java.util.Map.Entry;
import java.math.*;
import java.awt.geom.*;
import java.io.*;
      
      
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 yL = 0;
		double yR = 1001;
		while(yR - yL >= 0.00000001) {
			
			double yPA = ((yL + yR) / 2.0 + yR) / 2.0;
			double disAPA = Math.sqrt(Math.pow(xa, 2) + Math.pow(ya - yPA, 2));
			double disPBA = Math.sqrt(Math.pow(xb, 2) + Math.pow(yb - yPA, 2));
			double DIS2 = disAPA + disPBA;
			
			double yPB = (yL + (yL + yR) / 2.0) / 2.0;
			double disAPB= Math.sqrt(Math.pow(xa, 2) + Math.pow(ya - yPB, 2));
			double disPBB = Math.sqrt(Math.pow(xb, 2) + Math.pow(yb - yPB, 2));
			double DIS3 = disAPB + disPBB;
			
			if(DIS2 < DIS3) {
				yL = (yL + yR) / 2.0;
			}
			else {
				yR = (yL + yR) / 2.0;
			}
			
			
		}
		System.out.printf("%.20f\n",(yL + yR) / 2.0);
		
	}
	
}