int rightDiagonal = 0; int secondaryDiagonal = 0;

    int n = arr.size();

    for (int i = 0; i < n; i++) {
        rightDiagonal += arr.get(i).get(i);
        secondaryDiagonal += arr.get(i).get(n - i - 1);
    }

    return Math.abs(rightDiagonal - secondaryDiagonal);