using System;

public class Hello
{
    static void Main()
    {
        var n = int.Parse(Console.ReadLine().Trim());
        while (n >= 10) n = calc(n);
        Console.WriteLine(n);
    }
    static int calc(int a)
    {
        var res = 0;
        var b = a.ToString();
        foreach (var x in b) res += x - '0';
        return res;
    }
}