Total of Selected Items in Gridview

Question

Hello

I have a Gridview

I have Click Button and two labels. (Risk_label and MV_label)

Risk and MV column has price value.

My Checkbox column names are NameCheckBoxField1 and NameCheckBoxField2

How can i calculate only "Which i selected in Gridview" Risk total and MV total in my labels?

Example;

Risk (Checked rows) --> 10, 20, 30 ---> Risk_label = 60
MV (Checked rows) --> 20, 30, 40 ---> MV_label = 90

EDIT: I try this code;

double sum = 0;
foreach (GridViewRow gvr in GridView1.Rows)
{
    CheckBox cb = (CheckBox)gvr.FindControl("NameCheckBoxField1");
    if (cb.Checked)
    {
        double amount = Convert.ToDouble(gvr.Cells[1].Text);
        sum += amount;
    }
}
Risk_label.Text = sum.ToString(); 

But i getting an error.