import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int Px = sc.nextInt();
        int Py = sc.nextInt();
        int Qx = sc.nextInt();
        int Qy = sc.nextInt();
        int d = Math.abs(Px-Qx) + Math.abs(Py-Qy);
        double r = d/2.0;
        System.out.println(r);
    }
}