\int \!\!\!\int_D \int_{0}^{a-\sqrt{x^2+y^2}}dzdxdy=\int_{-\frac{\pi}{4}}^{\frac{3\pi}{4}}\int_{0}^{\frac{a}{\sqrt2}}\left(a-\sqrt{\left(\frac{a}{2}+\rho \cos \theta\right)^2+\left(\frac{a}{2}+\rho \sin \theta\right)^2}\right)\rho d\rho d\theta+\int_{0}^{a}\left(\int_{a-x}^{\sqrt{a^2-x^2}}\left(a-\sqrt{x^2+y^2}\right) dy\right)dx