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();
        double ans = (Math.abs(px - qx) + Math.abs(py - qy)) / 2.0;
        System.out.println(ans);
    }
}