Index of Letters - sorting issue

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

namespace _9.Index_of_Letters
{
    class Program
    {
        static void Main(string[] args)
        {
            string str = Console.ReadLine();
            char[] myArray = str.ToCharArray();
            char[] allLetters = new char[26];
            allLetters[0] = 'a';
            for (int i = 1; i < allLetters.Length; i++)
            {
                allLetters[i] = (char)(allLetters[i - 1] + 1);
            }
            for (int i = 0; i < allLetters.Length; i++)
            {
                for (int k = 0; k < myArray.Length; k++)
                {
                    if (allLetters[i] == myArray[k])
                    {
                        Console.WriteLine(myArray[k] + " -> " + i);
                    }
                }
            }
        }
    }
}