C=A-B in my report. How do i make A=C in next row in my report?

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In my form Record1
A=Contract Cost
B= Amount paid
C=Balance
When I go to new record or when I come back to edit a record, I want "C"
Becomes equal "A" and My balance should be New "A" which was C in prevoius
record.
I hope I am explaining myself.
Thanks you all
Tony