need formula to show positive withdrawal as "0"

I am finishing up a retirement calculator and have the situation where my formula is computing how much of the 401K withdrawal to subtract from the beginning 401k balance the next year. As an example, my beginning 401K balance in year 2 = end 401K balance year 1 minus 401k withdrawal in year 1. This works fine for years when there is an actual withdrawal. I am showing my withdrawals as (negative numbers) and "adding" that column back as described.

The problem is that in years where annual income exceeds retirement withdrawal needed, it computes a POSITIVE withdrawal. I do not know how to adjust the formula to add another condition so that when r21>0, it should not be added.

column g= end 401k balance
column r = 401k withdrawal
yr1= line 21

beg 401k balance yr2(line 22)=IF(ISNUMBER(R21),(G21+R21),G21)

Thanks

Screenshot attached.
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