Order of the symmetry group of Feynman Diagrams

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Hello,

I am learning Feynman diagrams and I still do not understad quite well the symmetry factor idea. The equation is:

$$ \frac{1}{O(G)} = \frac{M}{n!(4!)^n} $$

I was trying the next example:

If I am not wrong it is O(G) = 10 taking care of the asymmetry of each pair of internal lines connecting vertices.

 

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Now, I count S = O(G) = 12.

 

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Can anyone at least explain "for dummies" how to count the order of symmetry?