Run Length Encoding in C#

rle.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
//Write a C# program to implement the below compress the given string using Run Length Algorithm 

namespace RunLengthEncoder
{
    class Program
    {
        static void Main(string[] args)
        {
            //Declare the variables


            String S1;
            
            String S2="";
            int i;
            int c=1;             
            Console.WriteLine("Run Length Encoder");

            Console.WriteLine("Enter a String");

            S1 = Console.ReadLine();

            Console.WriteLine("You Entered : " + S1);
            S1 = S1 + " ";
            
            //Console.ReadLine();

            //Compress Using Run Length Encoding 

            for (i=0;i<S1.Length-1;i++)
            {
                   if (S1[i]==S1[i + 1])
                {      //When same increment counter
                    c++;
                }
                else {
                    
                    S2 += S1[i];
                    S2 += c;
                    c = 1;           
                }
                  
            }
            Console.WriteLine("Output" + "\n" + S2);

            Console.ReadLine();
        }
    }
}

How can this be modified to work when one letter is entered to give the output of A1?