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

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