Internship Online Round Problem

Given an Array of n elements s.t sum of ith and n-ith element is same for all i. (considering 1 indexed here).Then one element was removed from this array.

Now you are presented with n-1 elements and you need to find that missing element.
How will you do it?? (range of N is -> 10^5)

(Please give code as well if u can) :)

Thanks in Advance :)