Boundary Values & Fourier Series · Easy problem

The Fourier series of a $2\pi$-periodic function is written $\dfrac{a_0}{2} + \sum_{n\ge1}(a_n\cos nx + b_n\sin nx)$ with $a_n = \dfrac{1}{\pi}\displaystyle\int_{-\pi}^{\pi} f(x)\cos nx\,dx$. For $f(x) = |x|$ on $[-\pi, \pi]$, find $a_0$.