class Solution {
    public int diagonalSum(int[][] mat) {
        int res = 0;
        int n = mat.length;

        for (int i = 0; i < n; i++) {
            res += mat[i][i];
            res += mat[i][n - 1 - i];
        }

        return res - (n % 2 == 1 ? mat[n / 2][n / 2] : 0);
    }
}