Generate next ID's with prefix 'AE' to label C# to sql

Question

If I delete in between one record in master table, next auto generate id gives duplicate value

how to solve this 

 

string empid = "AE000"; 

 

con.Open();

SqlCommand cmd = new SqlCommand("select  Count(REGISTRATION_ID) from CANDIDATE_MASTER", con);

int i = Convert.ToInt32(cmd.ExecuteScalar());

con.Close();

i++;

Label1.Text = empid + i.ToString();

 

 

REGISTRATION_ID

AE0002

AE0004

AE0005

AE0006

 

If i execute that code ,label will get the value like 'AE0005'

i want to generate next id like 'AE0007'