We use the following result: when $f$ is a radial integrable function, that is, we can write $f(x)=g(\lVert x\rVert)$ where $\lVert \cdot\rVert$ is the Euclidian norm, we have $\int_{\Bbb R^n}f(x)dx=nV_n\int_0^{+\infty}r^{n-1}g(r)dr,$ where $V_n$ is the volume of the unit ball of $\Bbb R^n$ for the Euclidian norm.
To see that, we can show it when $f$ is a finite sum of maps of the form $a_j\chi_{A_j}$, where $A_j=\{x\in\Bbb R^n, |x|\in B\}$ and $B$ is a Borel subset of the real line. Then we use an approximation argument.
Back to the problem: let $f(x):=|x|^{-2np}\chi_{x,|x|\geq 1}$. It's a radial function.