Astrological Digits

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace AstrologicalDigits
{
    class AstrologicalDigits
    {
        static void Main(string[] args)
        {
            string n = Console.ReadLine();
            ulong sum = 10;
           /* if (n.StartsWith("-"))
            {
                n = n.Remove(0, 1);
            }*/
            while (sum > 9)
            {
                sum = 0;
                for (int i = 0; i < n.Length; i++)
                {
                    switch (n[i])
                    {
                        case '1': sum = sum + 1; break;
                        case '2': sum = sum + 2; break;
                        case '3': sum = sum + 3; break;
                        case '4': sum = sum + 4; break;
                        case '5': sum = sum + 5; break;
                        case '6': sum = sum + 6; break;
                        case '7': sum = sum + 7; break;
                        case '8': sum = sum + 8; break;
                        case '9': sum = sum + 9; break;
                    }
                }
                n = sum.ToString();
            }
            Console.WriteLine(sum);
        }
    }
}