Add columns

Question

i have student  table and student_id is primary key ,there is another column adm_no and i want when i admit new student adm_no increment by one without skipping i.e 1001 ,1002 etc 

int x = 0;

x = x + 1;

tStudent.adm_no = Convert.ToInt32(txtadm.Text) + x;

i have the above code but not working .

any assistant will be highly appreciated