Why complexity of nested loop? This is how I solved it:

long sum1 = 0; long sum2 = 0;

        for (int i = 0; i < n ; i++)
        {
            sum1 += a[i][i];
            sum2 += a[i][n - 1 - i];
        }
        Console.WriteLine(Math.Abs(sum1 - sum2));