Please do not use special functions and explain your solution, thanks. (Python)

Write a program that prompts the user for an integer, calculates the factorial of the input integer and displays the result as a string with a comma at every third position, starting from the right. Assume that user input is valid. Hint: Think about representing a number as a string. Sample Run 1: Sample Run 3: Enter an int: 0 0! = 1 Enter an int: 10 10! = 3,628,800 Sample Run 2: Sample Run 4: Enter an int: 4 4! = 24 Enter an int: 20 20! = 2,432,902,008,176,640,000