Sum equal to Kronecker delta - how?

I am to show that the following is true:

$\displaystyle \frac{1}{N}\sum_{g=0}^{N-1}exp\left( 2 \pi i (l'-l) \frac{g}{N}\right)=\delta_{l'l}$

I understand that for $\displaystyle l=l'$ it's equal to $\displaystyle 1$, but how to I get zero for $\displaystyle l \neq l'$ ?

Also note that $\displaystyle l,l',N,g$ are integers

Thanks for the help!

Re: Sum equal to Kronecker delta - how?

This is a geometric progression, so use the formula $\displaystyle 1+a+\dots+a^{N-1}=\frac{a^N-1}{a-1}$.