public static int Persistence(long n)
        {
            // your code
            string s_n = n.ToString();
            int counter = 0;

            while(s_n.Length != 1)
            {
                s_n = Mult(s_n);
                counter++;
            }

            return counter;
        }

        public static string Mult(string number)
        {
            string s_mult = string.Empty;
            int mult = 1;

            foreach(char n in number)
            {
                mult *= int.Parse(n.ToString());
            }

            s_mult = mult.ToString();

            return s_mult;
        }