private void btnAtualizar_Click(object sender, EventArgs e)
{
float b1 = 0;
dgvVenda.DataSource = vend.ListarVendas();
for (int i = 0; i < dgvVenda.Rows.Count; i++)
{
b1 += Convert.ToInt32(dgvVenda.Rows[i].Cells[4].Value);
}
lblTotal.Text = b1.ToString();
}