Alert Message with condition

Hi!


I need to receive always this alert message that says: "you limit is passed".
I write this code just now and when I make substraction. I need when the result is "egal" or "more than" 6000 km its should say that alert message in top: 

if (string.IsNullOrEmpty(lastKM.Text))
            {
                return;
            }
            double lastKM;
            double recenteKM;
            double ResultKms;
           lastKM= Convert.ToDouble(txtlastKM.Text);
           recenteKM= Convert.ToDouble(txtrecenteKM.Text);
            ResultKms = lastKM- recenteKM;
            lblResultKms.Text = ResultKms.ToString();
        }