Use Gauss' Lemma to compute

I know how to start this question but just unsure how to finish it, my working is as follows:

where is the number of negative least residues of the integers

the set of least residues mod 37 is

Does that mean I have to check through all of the integers in that set ( ) to see if they have a negative least residue in the set ? Because that would take a VERY long time, is there a faster way?

Cheers

Tonio