FrmAsyncAwait.cs

private async void btnCalcular_Click(object sender, EventArgs e)
{
    this.btnCalcular.Enabled = false;
    var tarea = Task<int>.Factory.StartNew(() => FuncionSumaLenta(Int32.Parse(txtNumeroA.Text),Int32.Parse(txtNumeroB.Text)));

    await tarea;

    this.lblResulado.Text = "Resultado: " + tarea.Result.ToString();

    this.btnCalcular.Enabled = true;
}