add a DataGridViewCheckBoxColumn to your DataGridView at relevant position.
in buttonCalucate_Click event.. loop through all the rows as follows and do total of the selected rows.
double dblTotal = 0.0;
foreach (DataGridViewRow row in dataGridView1.Rows)
{
DataGridViewCheckBoxColumn colObj = row.Cells[0] as DataGridViewCheckBoxColumn ;
if (colObj.Selected)
{
dblTotal += convert.toDouble(row.Cells[1].Value.toString());
}
}