Boundary Values & Fourier Series · Easy problem

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