import java.util.Scanner;
public class Main {
public static void main(String[] args) {
Scanner sc=new Scanner(System.in);
int n=sc.nextInt();
int m=sc.nextInt();
int l=sc.nextInt();
int r=sc.nextInt();
System.out.println(Math.max(Math.abs(n-l),Math.abs(m-r)));
}
}