how to get string before the second slash as a parameter??

Thank you my friends , But I think this way not work!

Because...

I know how to get url and it's segments by this class(URI)!

But I don't know how to change url analysing in codeigniter. In another words I want to codeiginter get first parameter as a $_GET parameter and second parameter as a controller and third parameter as a method and next parameter as another parameters.

Like this example:

http://hamresan.ir/intstitution1/search/add/ ... (and others as parameters)
"institution1" as a parameter like center (center=institution1)
"search" as a controller
and "add" as a method

But by default setting of codeigniter I have to get like this, but I don't want:

http://hamresan.ir/intstitution1/search/add/

"institution1" as a controller
"search" as a method
and "add" as a parameter

But I don't want this

please help me..