import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		double px = sc.nextDouble(); double py = sc.nextDouble();
		double qx = sc.nextDouble(); double qy = sc.nextDouble();
		double ans = Math.abs(px - qx) + Math.abs(py - qy);
		System.out.println(ans / 2.0);
	}
}