Count of Letters

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

class CountOfLetters
{
    static void Main()
    {
        string lettersString = Console.ReadLine();
        string[] allLetters = lettersString.Split(new char[] { ' ' }, StringSplitOptions.RemoveEmptyEntries);

        List<char> letters = new List<char>();

        for (int i = 0; i < allLetters.Length; i++)
        {
            letters.Add(Convert.ToChar(allLetters[i]));
        }

        letters.Sort();

        int counter = 1;
        for (int i = 1; i < letters.Count; i++)
        {
            if (letters[i] == letters[i - 1])
            {
                counter++;
            }
            else
            {
                Console.WriteLine("{0} --> {1}", letters[i - 1], counter);
                counter = 1;
            }

            if (i == letters.Count - 1)
            {
                Console.WriteLine("{0} --> {1}", letters[i], counter);
            }
        }
    }
}