Find the Sum

There is a secret sequence $A_1, A_2, \ldots, A_N$. We do not know the elements of this sequence, but we know another sequence $B_1, B_2, \ldots, B_{N-1}$ such that $B_i = A_i + A_{i+1}$ for each valid $i$. You should process $Q$ queries. In each query, you are given two indices $p$ and $q$; your task is to compute $A_p + A_q$ or determine that there is not enough information to uniquely determin