Opposite Cites

There are $N$ cities on a circle, numbered $1$ through $N$. For each $i$ ($1 \le i \le N-1$), cities $i$ and $i+1$ are directly connected by a bidirectional road with length $A_i$, and cities $N$ and $1$ are also directly connected by a bidirectional road with length $A_N$. However, we do not know the lengths of some roads. For each city $i$, we do know that it has an *opposite city* — formally,