C#: Count a total number of vowel or consonant

using System;

// Define the Exercise9 class
public class Exercise9  
{  
    // Main method - entry point of the program
    public static void Main() 
    {
        string str; // Declare a string variable to hold user input
        int i, len, vowel, cons; // Declare variables for iteration, string length, vowel count, and consonant count
	
        // Prompt the user to input a string
        Console.Write("\n\nCount total number of vowel or consonant:\n");
        Console.Write("----------------------------------------------\n"); 	
        Console.Write("Input the string: ");
        str = Console.ReadLine(); // Read the input string from the user		

        vowel = 0; // Initialize vowel count to zero
        cons = 0; // Initialize consonant count to zero
        len = str.Length; // Get the length of the input string

        // Loop through each character of the string to count vowels and consonants
        for (i = 0; i < len; i++)
        {
            // Check if the character is a vowel (both lowercase and uppercase)
            if (str[i] == 'a' || str[i] == 'e' || str[i] == 'i' || str[i] == 'o' || str[i] == 'u' ||
                str[i] == 'A' || str[i] == 'E' || str[i] == 'I' || str[i] == 'O' || str[i] == 'U')
            {
                vowel++; // Increment the vowel count
            }
            // Check if the character is an alphabet (both lowercase and uppercase) but not a vowel
            else if ((str[i] >= 'a' && str[i] <= 'z') || (str[i] >= 'A' && str[i] <= 'Z'))
            {
                cons++; // Increment the consonant count
            }
        }

        // Display the total number of vowels and consonants in the input string
        Console.Write("\nThe total number of vowels in the string is: {0}\n", vowel);
        Console.Write("The total number of consonants in the string is: {0}\n\n", cons);
    }
}