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: