How to number two-line function definitions

Question

Lets say we have a two line function definition, for example, a translation

\begin{align}
    t_{p,q}\text{ : }&\mathbb{R}^2 \longrightarrow \mathbb{R}^2 \\
    &(x,y) \longmapsto (x+p, y+q) \notag
\end{align}

What/how is the correct way of numbering this? I personally just number the first line, but is it possible to have this number vertically centred? Or is there a better way?