How do I sum four columns in vb.net... It's just concatenating the values instead

In my vb.net codebehind, I'm trying to sum four text fields that users submit integers.  Instead of getting a sum, it's just concatenating the values onto one another.  What am I doing wrong? 

Dim tt As Integer tt = txtConferenceRegistration.Text & txtHostingSeminar.Text & txtSponsorship.Text & txtMembershipDues.Text txtTotal.Text = tt