Allocation a number to specific column

[shahsavand]

i want to allocate the extra quantity of column G to one of the (A to D) if they filled with a number.i just wanna add the extera number to one of those column not all of them.
i would be very grateful if anybody could help me with a VBA code.

XS4020-34403	XS4020S-34503	XS5020-34603	XS5020S-34702	TOTAL NEED	Total RECEIVE	EXTRA
1000	1000	1000	1000	4000	4012	12
1000	1000	1000	1000	4000	12000	8000
1000	1000	1000	1000	4000	4216	216
2000	3000	2000	3000	10000	18000	8000
1000	1000	1000	1000	4000	12000	8000
0	1000	0	1000	2000	4000	2000
0	1000	0	1000	2000	2208	208
1000	0	1000	0	2000	2208	208
0	1000	0	1000	2000	10000	8000
1000	0	1000	0	2000	2304	304
0	1000	0	1000	2000	2560	560
0	1000	0	1000	2000	4000	2000
0	2000	0	2000	4000	6000	2000
1000	0	1000	0	2000	2940	940
0	1000	0	1000	2000	10000	8000
1000	0	1000	0	2000	5000	3000
0	1000	0	1000	2000	3040	1040
2000	2000	2000	2000	8000	40000	32000

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