Maximum number of distinct anagrams

I’m doing an online class learning c# and the task i have been assigned is to make a program that give you the maximum possible number of anagrams without repeating from a string of letters. The code i made works for the examples like abc , aabc, but when a large string is entered for example abcdefghijklmnoqprstuvwxyzabcdefghijklmnoqprstuvwxyz (alphabet twice) which is 52!, the result is 8.065829222532113E+26, where the result that the platform accepts is 1.20190046982384E+60. I am thinking that Aparitii function formula is not ok but i have no ideea how to make it work properly. It should be n1!n2!..nk! with n being the number of times a letter apeared and k the number of distict letters.

EDIT: the Aparitii function should find the factorial of the total distinct letters, for example: if a character is already present it should increment its factorial so at the end instead of aabc being factorial of 4! it should be 4! / 2!1!1!

using System;

public class anagram
{
    static void Main(string[] args)
    {
        string str = Console.ReadLine();
        double rez1 = NrTotalLit(str);
        double rez2 = Aparitii(str);

        Console.WriteLine(rez1 / rez2);
        Console.ReadKey();
    }

    static double NrTotalLit(string str)
    {
        double x = 1;

        for (int i = 1; i <= str.Length; i++)
        {
            x *= i;
        }
        return x;
    }

    static double Aparitii(string str)
    {
        double y = 1;
        double ap1 = 1;
        
        for (int i = 0; i < str.Length; i++)
        {
            for (int j = i + 1; j < str.Length; j++)
            {
                if (str[i] == str[j])
                {
                    y = (ap1 *= j) + j;    // this formula might not be good    
                }
               
            }
        }
        return y ;      
    }
}