I found a formula for the **degree of unsaturation (DU)** in **Clayden’s Organic Chemistry**. It explains, simply, the unsaturation with the difference in H atoms. Then I stumbled upon the [general formula](https://en.wikipedia.org/wiki/Degree_of_unsaturation):

$$DU=1+\frac12\sum n_i\left(v_i-2\right)$$

$n_i$ is the number of atoms with valence $v_i$.

Can someone explain from where this equation was derived? And how come the valence term was introduced?