Maple code for series

Question

With thanks in advance

I have problem in writting Maple code for : $$\sum_{\underset{\large(N+i)\ \ odd}{i=0}}^{N-1}a_{i} $$

where $N‎ \geq ‎1 $ is input.

With regards

Answer 1

sum(a[N-2i-1], i=0..floor((N-1)/2))

Edit: and for $N+i$ even:

sum(a[N-2i], i=0..floor(N/2))