class Program
{
    static void Main(string[] args)
    {
        string input = Console.ReadLine()!;
        int sum = 0;

        while (input.Length > 1)
        {
            foreach (char c in input)
            {
                string num = c.ToString();
                sum += int.Parse(num);
            }
            input = sum.ToString();
            sum = 0;
        }

        Console.WriteLine(input);
    }
}