How can I apply the definition of continuity to the following exercise to find the points where $f$ is continuous?
$$f\left ( x,y \right )=\left\{\begin{matrix} x^{2}+y^{2}, \textrm{if}\,\, x^{2}+y^{2}\leq 4\\ 1\: \: \: \: \: \: \: \: \: \: \: \: , \textrm{if}\,\, x^{2}+y^{2}> 4 \end{matrix}\right.$$
Thanks for your help