import java.util.*;
import java.math.*;

class Question410
{
	public static void main(String[] str){
		
		Scanner scan = new Scanner(System.in);
		
		double px = scan.nextDouble(), py = scan.nextDouble();
		double qx = scan.nextDouble(), qy = scan.nextDouble();
		
		System.out.println((Math.abs(px-qx)+Math.abs(py-qy))/2);
	}
}