The reason of your problem is that you mix up, and seem to miss the difference, between numbers and digit characters. When you do odd = [1,3,5,7,9], then the list odd contains 5 numbers: 1, 3, 5, 7, and 9. When you do number = list(str(sum(arr))), the number list holds characters, for example: '1', '3', '5', '7', or '9'. That's why when you do number[-1] in odd you get no results: because for example '5' is not in [1, 3, 5, 7, 9].

5 is, but '5' is not. You need to find out what's the difference between '1' and 1, or between '7' and 7.

Thank you for the solution

Because it is not correct.

Without seeing your solution we do not know why it is not correct.

Why my solution doesn't work?

Thank you Polarish, that's important!