I'm having some trouble in the following proof:

Let $f:\mathbb{R\to R}$ be given by:
$$
 f(x) = \begin{cases}
1,&x\in\mathbb Q
\\
0,&x\notin\mathbb Q
\end{cases}
$$

Prove that $f$ is not continuous for any value in $\mathbb R$.

Can anyone please help me with this?

Thank you!