optimisation of gain function

little optimisation :
before, $Gain(S, A) = Entropy(S) - \displaystyle \sum_{v\ \in\
valeurs(A)}\frac{|Sv|}{|S|} \times Entropy(S_v)$.
Now, $Gain(S, A) = \displaystyle \sum_{v\ \in\
values(A)}\frac{|Sv|}{|S|} \times Entropy(S_v)$