How to Calculate the Sum of GridView Columns Using JavaScript?

I have a GridView with three COlumns Which will be like this:

 ID                  Sign                Amount
------              --------             ---------
 1                    +                   1000
 2                    -                    500
 3                    -                    750

So the Sum of the Column "Amount" Should be "-250". Consider the Column "Sign" also with the Amount. Here is my GridView's Source Code:

 <Columns> 
  <asp:BoundField DataField="ID" HeaderTe开发者_JAVA技巧xt="ID"/>
       <asp:TemplateField HeaderText="Sign" >
        <ItemTemplate>
 <asp:TextBox ID="txtgvSign" runat="server" Text='<%# Bind("Sign") %>'></asp:TextBox>
</ItemTemplate>
</asp:TemplateField>
 <asp:TemplateField HeaderText="Amount">
  <ItemTemplate>
 <asp:TextBox ID="txtAmount" runat="server" Text='<%# Bind("Amount") %>'></asp:TextBox>  
</ItemTemplate>
</asp:TemplateField>
</Columns> 

So far, I have written One JavaScript, But It Only Calculate the Sum of Amount. I dont know how to calculate the Sum based on the Sign values. Below is the JavaScript I have written:

  function CalculateTax(fixedtotal)
  {
    var taxgrid = document.getElementById('<%=gvAttribute.ClientID %>');
    var taxip = taxgrid.getElementsByTagName('input');
    var taxamount = 0*1;

    for(i = 0; i < taxip.length; i++)
    {
        var tax = taxip[i].value;            
        taxamount = parseFloat(taxamount) + parseFloat(tax);
    }
    return parseFloat(fixedtotal) + parseFloat(taxamount);
  }

So Please Make Changes to this Javascript.

can you try the below code?

  function CalculateTax(fixedtotal)
  {
    var taxgrid = document.getElementById('<%=gvAttribute.ClientID %>');
    var taxip = taxgrid.getElementsByTagName('input');
    var taxamount = 0*1;

    for(i = 0; i < taxip.length; i+= 2)
    {
        var sign = taxip[i].value;            
        var tax = taxip[i+1].value; 
        taxamount = parseFloat(taxamount) + (sign =='+' ? 1 : -1)* parseFloat(tax);
    }
    return parseFloat(fixedtotal) + parseFloat(taxamount);
  }

<script language="javascript" type="text/javascript">
    function Calculate() {

        var grid = document.getElementById("<%=grid.ClientID%>");
        var sum = 0;
        for (var i = 1; i < grid.rows.length; i++) {
            var Cell = grid.rows[i].getElementsByTagName("input");
            if (!Cell[4].value) {sum += 0; } else { sum += parseFloat(Cell[0].value);}
        }

        alert(sum);        
    }

</script> 



<asp:TemplateField HeaderText="Current payment" >
<ItemTemplate>
    <asp:TextBox ID="txtvalue" runat="server" Width="70px" BorderStyle="None" onkeyup="Calculate();" ></asp:TextBox>
</ItemTemplate>
<ItemStyle Width="120px" />
</asp:TemplateField>`enter code here`