Add Value to RoutValues and keep Current Values in ASP.NET MVC 3

I have a view with two form one of them for declare rows per page. In post action I need my Url be the same and just add new parameter. at now I use like this:

[HttpPost]
 public ActionResult Index(FormCollection collection) {
 
     //Calculate Row Count
 
     return RedirectToAction("Index", new { RC = RowCount });
 
    }
 

with this implementation my all parameters lost and just RC = rowcountnumber replaced.
How can I keep parameters and just add new RC to it? what is fastest way to do it? is any performance issue here?

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