Formula for Inventory Sheet Not Working

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Can someone help me create the formulas for a spreadsheet I created. I am new to formulas and have done some of this. I am creating this for a volunteer position at a senior citizens center and I am getting really frustrated. I have attached the sheet I am working with. My main problem is how to create a formula that will figure the current months usage dollars (column M). I need to first use up the inventory on hand Column G at Column F's cost and then add in items used from the current months cost Column J. I don't seem to be able to upload an attachment so I could email it to someone willing to help.


Any help or advice is appreciated.

Thanks!

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Here is an attachment of spreadsheet.

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=(F4*E4)+(K4-F4)*I4 is the formula I am using in column M. It seems to work in first row but not second. Any advice is appreciated - I am new to this!