I have to understand the idea of the index calculus algorithm. The algorithm can be found here.

We have to find the discrete logarithm of an element of a multiplicative group of intergers modulo n (say n=5), let's say to base 2 (2 is a generator of $\displaystyle \mathbb{Z}_5^*$).

We need a factor base in the input. The factor base is a tuple $\displaystyle (-1,2,3,5,7,11,...,p_r)$, where $\displaystyle p_r$ is the r-th prime. My question is why do we want -1 in the factor base?