Write a Program in C# to add the given number till it get Single digit

using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

using System.Threading.Tasks;

namespace ADDINTTILLONEDIGIT

{

    class Program

    {

        static void Main(string[] args)

        {

        start:

            Console.WriteLine("Enter the integers to make it to single int");

            string input = Console.ReadLine();

            if (!input.All(char.IsDigit))

                goto start;

            else

            {

                while (input.Length > 1)

                {

                    input = input.Sum(x => x - '0').ToString();

                }

                Console.WriteLine("Single digit number is {0} :-)",input);

                Console.ReadKey();

            }

        }

    }

}

Output: