If $\displaystyle p$ and $\displaystyle q$ are positive integers such that $\displaystyle \frac{p}{q} = 1 + \frac{1}{2} - \frac{2}{3} + \frac{1}{4} + \frac{1}{5} - \frac{2}{6} + \frac{1}{7} + \frac{1}{8} - \frac{2}{9} + \mbox{...} + \frac{1}{478} + \frac{1}{479} - \frac{2}{480}$, show that $\displaystyle p$ is divisible by 641.