Example: Push-Forward Sheaf

Question

Let $f: X\to Y$ be a continuous map, $\mathscr{F}$ a sheaf on $X$.
$f_*\mathscr{F}$ is the sheaf on $Y$ defined by $f_*\mathscr{F}(U)=\mathscr{F}(f^{-1}(U))$ Uand $\rho_{VU}=\rho_{f^{-1}(V)f^{-1}(U)}: f_*\mathscr{F}(V)\to f_*\mathscr{F}(U)$.

Example: Let $X= \{P,Q\}$ with the discrete topology and $Y=\{R\}.$ $f:X\to Y$ and let $\underline{\mathbb{R}}$ be the sheaf of local constant real valued functions on $X$.
Then $ f_*(\underline{\mathbb{R}})=\left\{\begin{array}{ll} \mathbb{R}^2, & U=\{R\} \\ 0, & U=\emptyset\end{array}\right.$

Why do we have that $\underline{\mathbb{R}}(f^{-1}(R))= \mathbb{R}^2$?
I don't see where this comes from.