Thank you, Antonio!

the best

It was stated above in the comments, but to save you some time...

"Because the requirement asks "How many years does the town need to see its population p0 >= p inhabitants?", so you need to repeat the increase of population until p0 < p.
If you repeated the while until p0 <= p, you'd get an extra year in the result (e.g. if p0 = p, you'd get 1, but the expected value would be 0!)."

shouldn't it be <= instead of <?

[] 20 [10, -80, 10, 10, 15, 35] -> left_sum = 0, right_sum = 0, index_value = 20, index=1
[10, -80, 10, 10, 15, 35] 20 [] -> left_sum = 0, right_sum = 0, index_value = 20, index=6

thanks! I got it!

Try reading the posts below, razorpl's one for example.