Range Query with conditions

Question

Suppose I have an $N$ length integer array of pairs of the type $[value, key]$. Now, I need to query for range sum. Query is of the type : $l, N, x$ meaning I have to sum up all the $value$ in the index range $l$ to $N$ having key as $x$. Also, there can be updates that add a new pair to the end of the array increasing $N$. Succeeding queries consider the updated $N$. There can be multiple queries so I need to answer the query in $O(logN)$ but precomputation is allowed.

I think it might be done with a Segment Tree but I cannot see what to keep in the nodes. Is it even possible to do it with required complexity, because I feel you actually need to go to every element in the array to check it's key. Any help is appreciated.