How to do calculations in grid view adding subtracting

Question

Name fixedpayment june2015 july2015 August2015 balancepay
madu 6000
Naresh 6000
Suman 6000
Srinu 6000


i have fixed payment value is 6000 if I give value 6000 in june2015 show balance pay 0, if i give value 5000 in july2015 show balancepay 1000.
i want to show like this when i update my grid view

Name fixedpayment june2015 july2015 August2015 balancepay
madu 6000 5000 1000
Naresh 6000 6000 0
Suman 6000 2000 4000
Srinu 6000



<asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="false" DataKeyNames="id"
OnPageIndexChanging="GridView1_PageIndexChanging" OnRowCancelingEdit="GridView1_RowCancelingEdit"
OnRowDeleting="GridView1_RowDeleting" OnRowEditing="GridView1_RowEditing" OnRowUpdating="GridView1_RowUpdating">

<asp:BoundField HeaderStyle-Width="50px" DataField="Id" HeaderText="ID" />
<asp:BoundField HeaderStyle-Width="200px" DataField="Name" HeaderText="Name" ItemStyle-CssClass="ContactName" />
<asp:BoundField HeaderStyle-Width="150px" DataField="ActualPay" HeaderText="Actual Pay " />
<asp:BoundField HeaderStyle-Width="150px" DataField="June2015" HeaderText="June 2015" />
<asp:BoundField HeaderStyle-Width="100px" DataField="July2015" HeaderText="July 2015" />
<asp:BoundField HeaderStyle-Width="100px" DataField="August2015" HeaderText="August 2015" />
<asp:BoundField HeaderStyle-Width="125px" DataField="September2015" HeaderText="September 2015" />
<asp:BoundField HeaderStyle-Width="125px" DataField="Total" HeaderText="Balance Pay" />
<asp:CommandField ShowEditButton="true" />
<asp:CommandField ShowDeleteButton="true" />





i have done edit ,update , delete for grid view
how to do that above calculations