Suppose f \in R(x) on [0,1]. Define a_n = \frac{1}{n} \sum_{k=1}^n f\left(\frac{k}{n}\right) for all n. Prove \{a_n\}_{n=1}^{\infty} converges to \int_0^1 f ~dt

I missed the class where they covered Riemann Sums. If I can get some help starting this off I would be grateful.