Create Loop In A Defined Range

i created list in Excel and i wrote into (a1 cell-income, b1 cell-expence, c1 cell-kredit and d1 cell-debet). i enter a2 cell income (for ex:1000) and b2 cell expence (for ex: 100) and I give condition in VBA that if income greater than expence then VBA writes a2.value-b2.value into d2 cell else if expence greater than income then it writes b2.value-a2.value into c2 cell but i only define this one row (for ex: c2, a2. b2. d2).

i would like to assign it the rows as long as i want
that is i would like to create loop
(for ex: a(i), b(i) and so on

here is