private void btnCalcular_Click(object sender, EventArgs e)
    {
        double total = 0;

        foreach (DataGridViewRow row in dataGridView1.Rows)
        {
            double a = Convert.ToDouble(row.Cells[0].Value);
            double b = Convert.ToDouble(row.Cells[1].Value);
            row.Cells[2].Value = a - b;
            total += Convert.ToDouble(row.Cells[2].Value);
        }
        label1.Text = total.ToString();
    }