using System;

using System.Linq;

public class Program

{

    public static void Main()

    {

        Console.WriteLine(CountLetterOccurrences("This is a string", 2));// return 3

        Console.WriteLine(CountLetterOccurrences("hello WORLD", 3));// return 1

        Console.WriteLine(CountLetterOccurrences("Life, the universe, and everything", 3));// return 3      

    }

    public static int CountLetterOccurrences(string input, int minOccurrences){

        return input.ToLower().ToList().GroupBy(x=>x).Where(x=> !char.IsWhiteSpace(x.Key) && x.Count() >= minOccurrences).Count();

    }

}