Asp.Net Not Hesaplama

<pre class="lang: decode:true "> protected void BtnCalculate_Click(object sender, EventArgs e)
        {

            double not1, not2, per1, per2, avg;

            not1 = Convert.ToDouble(TxtNot1.Text);
            not2 = Convert.ToDouble(TxtNot2.Text);
            per1 = Convert.ToDouble(TxtPer1.Text);
            per2 = Convert.ToDouble(TxtPer2.Text);

            av = (grade1 + grade2 + per1 + per2) / 4;
            LblOrt.Text = Convert.ToString(avg);

            if (avg < 50)
            {
                LblStatus.Text = "FAILED";

            }
            else if(avg>=50 && avg<70)
            {
                LblStatus.Text = "PASS";
            }
            else if (avg >= 70 && avg < 85)
            {
                LblStatus.Text = "THANK YOU CERTIFICATE";
            }
            else if (avg >= 85 && avg <= 100)
            {
                LblStatus.Text = "CERTIFICATE";
            }
        }</pre>