import java.util.Scanner;

public class Deai {

	public static void main(String[] args) {
		Scanner s = new Scanner(System.in);
		double Px = s.nextDouble(),Py = s.nextDouble(),Qx = s.nextDouble(),Qy = s.nextDouble();
		s.close();
		System.out.println((Math.abs(Px-Qx) + Math.abs(Py-Qy)) / 2);

	}

}