Program to find the sum of digits of a given number until the sum becomes a single digit. (e.g. 12345=>1+2+3+4+5=15=>1+5=6) .

class Program
    {
        static void Main(string[] args)
        {
            string num = Console.ReadLine();
            int sum = DigitSum(Convert.ToInt32(num));
            Console.WriteLine("sum:{0}", sum);
            Console.Read();
        }
        static private int DigitSum(int num)
        {
            int sum = 0;
            while (num > 0)
            {
                sum += num % 10;
                num /= 10;
            }
            if (sum > 9)
            {
                sum = DigitSum(sum);
            }
            return sum;
        }
    }