Write $\displaystyle \frac{1}{1} + \frac{1}{2} + ... + \frac{1}{p-1} = \frac{a}{b}$, with $\displaystyle gcd(a,b)=1$. Show that $\displaystyle p^2 $ divides a if $\displaystyle p \geq 5$

I have no clue how to express it as $\displaystyle \frac{a}{b}$.