import java.util.*;

class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int px = in.nextInt();
        int py = in.nextInt();
        int qx = in.nextInt();
        int qy = in.nextInt();

        double dx = 0.0;
        double dy = 0.0;

        if(px * qx < 0)
            if(px < 0)
                dx = qx - px;
            else
                dx = px - qx;
        else
            dx = Math.abs(px - qx);

        if(py * qy < 0)
            if(py < 0)
                dy = qy - py;
            else
                dy = py - qy;
        else
            dy = Math.abs(py - qy);

        System.out.println(dx/2 + dy/2);
    }
}