numbering rows in ALV report

hi

i have an alv report and i want to number the lines of of the alv table. i created a variablle which give the number to the line- i loop at the alv table and add 1 in each look and give the number to the row but..... when the alv report appers and i want to sort the table the column of row_number is also sorted and the numbers are not in the right order. is ther a way to number the rows and it wont be affected from sorting or filtering the table ?

thanks