import java.util.*;

public class Main {
    private static Scanner sc = new Scanner(System.in);
    public static void main(String[] args) throws Exception {
        float ax = sc.nextFloat();
        float ay = sc.nextFloat();
        float bx = sc.nextFloat();
        float by = sc.nextFloat();
        
        float ret = (Math.abs(ax-bx) + Math.abs(ay-by)) / 2;
        
        System.out.println(ret);
    }
}