public static int superDigit(string n, int k)
    {
        long start = 0;
        for(int i = 0; i < n.Length; i++)
        {
            start += (n[i] - '0');
        }
        start = start * k;
        return (int)backtrack(start);
    }
    static long backtrack(long num)
    {
        if(num <= 9) return num;
        long res = 0;
        while(num > 0)
        {
            res += num % 10;
            num = num / 10;
        }
        return backtrack(res);
    }