public static int superDigit(string n, int k)
    {
        long digitSum(string str)
        {
            long sum = 0;
            foreach (char c in str)
            {
                sum += c - '0';
            }
            return sum;
        }

        long initialSum = digitSum(n) * k;

        while (initialSum >= 10)
        {
            initialSum = digitSum(initialSum.ToString());
        }

        return Convert.ToInt32(initialSum);

    }