factorial zeroes

Program.cs

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

namespace PrimeFactorization
{
    class Program
    {
        static void Main(string[] args)
        {
            int n = 0;
            Console.Write("Enter a number n, or -1 to quit: ");
            string str = Console.ReadLine();
            n = int.Parse(str); //convert string to int
            while(n >= 0)
            {                
                CountZeroesAtEndOfFactorial(n);
                Console.Write("Enter a number n, or -1 to quit: ");
                str = Console.ReadLine();
                n = int.Parse(str); 
            }            
        }

        private static void CountZeroesAtEndOfFactorial(int number)
        {
            List<int> factors = GetPrimeFactorizationOfFactorial(number);
            int twos = 0, fives = 0;
            foreach (int num in factors)
            {
                if (num == 2)
                    twos = twos + 1;
                if (num == 5)
                    fives = fives + 1;
            }
            int zeroCount = (twos < fives) ? twos : fives; //get the smaller quantity

            Console.WriteLine("{0}! has {1} zeroes at the end", number, zeroCount);
        }

        private static List<int> GetPrimeFactorizationOfFactorial(int num)
        {
            List<int> list = new List<int>();

            if (num < 2)
            {
                list.Add(num);
                return list;
            }

            for (int i = 2; i <= num; i++)
                list.AddRange(GetPrimeFactorization(i));

            return list;
        }

        private static List<int> GetPrimeFactorization(int num)
        {
            List<int> list = new List<int>();

            if (IsPrime(num))
            {
                list.Add(num);
                return list;
            }

            int i = 2;
            while (i * i <= num)
            {
                if (num % i == 0)
                {
                    int result = num / i;

                    if (IsPrime(result))
                        list.Add(result);
                    else
                        list.AddRange(GetPrimeFactorization(result));

                    if (IsPrime(i))
                        list.Add(i);
                    else
                        list.AddRange(GetPrimeFactorization(i));

                    return list;
                }
                i++;
            }
            return list;
        }

        private static bool IsPrime(int num)
        {
            if (num == 1)
                return false;
            if (num == 2)
                return true;

            int i = 2;
            while (i * i <= num)
            {
                if (num % i == 0)
                    return false;

                i++;
            }
            return true;
        }
    }
}