How to sum of two rows values inside the datagridview c#

How to sum of two rows values inside the datagridview, if the id of the both rows same  and create one rows instead of two rows in the datagrid

or

and for the multiple rows values

like the following example:

id  quantity  Bonus  price  total

01  25  10  123  4305

01  5  10  123  1845



i want to sum of the above two rows like this in the datagrid

id  quantity  Bonus  price  total

01  30  20  123  6150

if i take loop on it then duplicate value insert in the datagridview.

like my code

For example:

for(int i=0; i{

         if(datagridview.rows[i].cells[0].value==txtid.Text)
        {
                if(comboboxtype=="Stock")
                        {
                                 //For Stock
                                ///updation code,if value exist in the datagrid
                               }
                        else
                        {
                                         //For Bonus
                                ///updation code,if value exist in the datagrid
                               }

        }
        else
        {
                if(comboboxtype=="Stock")
                        {
                                        //For Stock

                                ///  ///Insertion code, if value does not exist in the datagrid
                               }
                        else
                        {
                                        /// For Bonus
                                ///  ///Insertion code, if value does not exist in the datagrid
                               }
               

        }
}

plz, Anyone can help me ..............