Multiple Users accessing same variable

See more:

ASP

ASP.NET

I've hosted an Asp website in vb.net . website is all about calculating average, when two user opens my website and tries to make any calculation it shows RUNTIME error. When a single user uses the website it works properly. What would be the solution.

Am using using combo box listed items in combo box are A,B,C . each items holds value like 10,9,8.
Calculate button to find the average.
when to users uses same combo box it results error.
http://www.aug.somee.com/[^]
this is my website .

Dim tot1 As UInt32

Public Sub Button1_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles Button1.Click
        tot1 = 21
        If DropDownList1.SelectedValue = 0 Then
            tot1 = tot1 - 3
        End If
        If DropDownList10.SelectedValue = 0 Then
            tot1 = tot1 - 3
        End If
        If DropDownList11.SelectedValue = 0 Then
            tot1 = tot1 - 3
        End If
        If DropDownList12.SelectedValue = 0 Then
            tot1 = tot1 - 4
        End If
        If DropDownList13.SelectedValue = 0 Then
            tot1 = tot1 - 4
        End If
        If DropDownList14.SelectedValue = 0 Then
            tot1 = tot1 - 2
        End If
        If DropDownList15.SelectedValue = 0 Then
            tot1 = tot1 - 2
        End If


        sem1 = ((DropDownList1.SelectedValue * 3) + (DropDownList10.SelectedValue * 3) + (DropDownList11.SelectedValue * 3) + (DropDownList12.SelectedValue * 4) + (DropDownList13.SelectedValue * 4) + (DropDownList14.SelectedValue * 2) + (DropDownList15.SelectedValue * 2)) / tot1
        TextBox1.Text = sem1

    End Sub