for (int i = 1; i <= 2 * n; i++)
        for (int j = 1; j <= 2 * n; j++)
            m[i][j] = (i % 2) * (i / 2 * 4 * n + 2 * j - 1) + !(i % 2) * ((i - 1) / 2 * 4 * n + 2 * (2 * n - j + 1));