Diagonal Difference

import java.util.*;


class Main {
    public static void main(String[] args) {
        List<List<Integer>> arr = new ArrayList<>();
        arr.add(Arrays.asList(1,2,3));
        arr.add(Arrays.asList(4,5,6));
        arr.add(Arrays.asList(9,8,9));
        
        int firstDiagonal = 0;
        int secondDiagonal = 0;
        int length = arr.size();
       
       for(int i = 0; i < length; i++){
           firstDiagonal += arr.get(i).get(i);
           secondDiagonal += arr.get(i).get(length - i - 1);
       }
        
        System.out.println(Math.abs(firstDiagonal - secondDiagonal));
    }
}