Sort chars in text and count them (Exercise)

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;

namespace ConsoleTests
{
    class ConsoleTests
    {
        static void Main()
        {
            string text = Console.ReadLine();
            char[] characters = text.ToLower().ToCharArray();
            Array.Sort(characters, StringComparer.InvariantCulture);
            int count = 1;
            for (int i = 0; i < characters.Length && i+1 < characters.Length; i++)
            {
                if (Char.IsLetter(characters[i]))
                {
                    if (characters[i] == characters[i + 1])
                    {
                        count++;
                    }
                    else
                    {
                        Console.WriteLine("{0} --> {1}", characters[i], count);
                        count = 1;
                    }
                }
            }
        }
    }
}