Why the last number of the serie become a negative integer?

Question

Supostly the output's numbers have to be always positive. I don't understand how a sum between 2 positive numbers is equal to a negative number.
Plz help 🙃

https://code.sololearn.com/ck6h6x1kF9kQ/?ref=app

Answer 1

check out this:
https://www.sololearn.com/discuss/1176730/?ref=app

Answer 2

Ok thank you. How I do that?

Answer 3

Thanks

Answer 4

didn't worked

Answer 5

I used what you wrote and the output is the same