Basic Chemistry problem

[maxpayne_lhp]

Well, hello!
I was informed a new formula. As a non-native, I can't give out the definitons, I hope that you can help me call some.
Well, here is the formula:
\Delta=\frac{2+\Sigma n_i (V_i-2)}{2}
Well, as far as I can say, \Delta here is equal to the total of the linkage C=C+ number of 'cycles' in my language?
So, what is this for and how was it formed? I really want to know so that I can apply to my problems.
Thanks for your time and help!

[maxpayne_lhp]

Ah, by the way, We have:
If \Delta =0, the subtance is not 'full' so; it has no \pi_{cc} linkage, no 'cycle.
If \Delta =1, the subtance has 1 \pi_{cc} linkage OR 1 'cycle'
If \Delta =2, the subtace has 2 \pi_{cc} linkages OR 1 \pi_{cc} linkage + 1 'cycle' OR 2 'cycles'
So, what is this all about?
Thanks again!