Tiles Still Needed

In a word game, you hold some letter tiles and want to spell a word. Each tile can only be used once.

Task: write missing_letters(tiles, word) that returns a dictionary of the letters you are still short of, with how many more of each you need.

• tiles and word are both text. Ignore capitals: treat everything as small letters.
• Only include letters you actually need more of. Don't include letters with a count of 0.
• If you can already spell the word, return {}.
• Return a plain dictionary.

You have one n but need two, and two as but need three. The b is covered.