How NDF is calculated

I have experimental data of number of samples, I plot histo and fit it by gaussian.
The problem is: I think that NDF should be calculated with formula
NDF=[Nb of bins] - [Nb of parameters] - 1
but it seems to be without [-1].
The questions are:
Who is right I or ROOT?
And how NDF is calculated?

Thanks for answering

According to Wikipedia:

The degree of freedom, ν = n - m, equals the number of observations n minus the number of fitted parameters m.

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