using System;
using System.Linq;

namespace CrockedDigits2
{
    class Program
    {
        static void Main(string[] args)
        {

            string input = Console.ReadLine();

            do
            {
               var result = (input.ToList().Where(x => Char.IsDigit(x)).ToList().Sum(x => x - 48).ToString());
               input = result;

            } while (input.Length > 1);

            Console.WriteLine(input);

        }
    }
}