I am trying to add a certain letter (H) to the end of a numerical sequence in a spreadsheet but cannot get it to populate on its own without changing the numerical sequence. For instance I have it in excel as
12G10000
12G10001
12G10002.. and so on but i need to the letter H so that it looks like

12G10000H
12G10001H
12G10002H.. and so on but when I add the letter and then do the usual highlight and drag down it changes the sequence so that all of the subsequent numbers are the same as the first highlighted cell.

any suggestions?