Shweta
The idea is simple. You can also easily crack it as its nothing advanced level computations.. For every entry in IT_FINAL you read the next entry and subtract stuff.
Loop at it_final into wa_final.
lv_tabix = lv_count = sy-tabix.
lv_count = lv_count + 1.
READ TABLE it_final into wa_final1 INDEX LV_COUNT.
if sy-subrc eq 0.
wa_final-menge1 = wa_final1-menge - wa_final-menge
Modify it_final index lv_tabix from wa_final transporting menge1.
endif.
endloop.