How to add constraint on the parameters?

Hi all

the following is the part of code,

RooRealVar c1(“c1”,“c1”,1000,0,10000000);
RooRealVar c2(“c2”,“c2”,2000,0,10000000);
RooRealVar c3(“c3”,“c3”,3000,0,10000000);
RooRealVar c4(“c4”,“c4”,3000,0,10000000);

RooAddPdf model(“model”,“model”,RooArgList (h2Pdf,h9Pdf,h1Pdf,h6Pdf), RooArgList(c1,c2,c3,c4));

model.fitTo(datad0,“e”);//extended likelihood model

if I want to add the constraint on the parameters ,such as c1+c2=c3
How to realize it?

any one could help me ?

I have dealed with the problem.
thx all.

how have you solved it?