Naive Approach

The naive solution looks like this: For each query $(l, r)$, loop from $l$ to $r$ and sum the elements. Simple and correct, but slow. If $n = 10^5$ and $q = 10^5$, you're doing up to $10^{10}$ operations. Most judges give you about $10^8$ operations per second.

Your code would take 100 seconds. Time limit is usually 1-2 seconds. The problem isn't its correctness. It's the repeated work. Every query recomputes sums from scratch. What if you precomputed something useful?