Compute $\iint \sqrt {\frac {1-x^{2}-y^{2}}{1+x^{2}+y^{2}}}\,\mathrm dx\mathrm dy $

Question

Compute $$\iint \sqrt {\frac {1-x^{2}-y^{2}}{1+x^{2}+y^{2}}}\,\mathrm dx\mathrm dy $$ and $x^{2}+y^{2}\leq 1$

I used polar coordinates then $\sqrt {\dfrac {1-r^{2}}{1+r^{2}}}r\,\mathrm dr\mathrm d\theta$, but couldn't continue.

Thanks for helping