How to fit using fixed values?

Good morning, I want to fit some data using a gaussian function f(x)=a1_1*exp(-((x-b1_1)/c1)^2 and fixing a1 and c1 values. Is it possible?

Ps. I can't just write the values in the function because I need to acquire the a1 and c1 values by keyboard during the run.

I need to do:

a1=input('a1= ');
c1=input('c1= ');
[myfitga1,god1]= fit(eVcIpicco.',ckxfitIpicco.','a1*exp(-((x-b1)/c1)^2)','StartPoint',[284.7]);

using in the fit the a1 and c1 fixed values

Thanks