Write a function to evaluate $f$ for any two user specified values $x$ and $y.$ The function $f$ is defined as:

$f=$

$x+y,x0$ and $y0$

$x+y^2,x0$ and $y<mn\; id="EditorElement\_3966324">0</mn>$

$x^2+y,x<mn\; id="EditorElement\_3966347">0</mn>$ and $y0$

$x^2+y^2,x<mn\; id="EditorElement\_3966391">0</mn>$ and $y<mn\; id="EditorElement\_3966405">0</mn>$