i have the words.txt file downloaded. i need to know how could i get python to return number of words in the file.

4. Write a function num_contains_only that takes a string of letters as input, and that returns the mumber of words in the file words.txt that contain only the letters in the given string of letters Inside your function, you should open the file, traverse its contents, and use the function contains only to get the count you want. Note that your function is just returning a mumber no printing of words is involved