How can I include a previously created variable in the definition of a TF1 function?

Hi all!
I want to make a fit on a TH1F histogram and the fitting function must be a linear combination of 3 normalized gaussians multiplied by a double variable (N in the example below) I’ve previously defined and calculated.
So I tried something like:

double N;
//...calculate N...
auto *f1 = new TF1("f1", "N*(gausn(0) + gausn(3) + gausn(6))",93.,110.); 

but, during the execution, it compares an error message saying basically that it doesn’t recognize N.
How can I solve this problem?
Thank you a lot!

TF1 *f1 = new TF1("f1", TString::Format("%.17g * (gausn(0) + gausn(3) + gausn(6))", N), 93., 110.);

It worked very well! Thank you very much!