Simple, yet not so simple formula help

I appreciate any help with this I can get.  I have a spreadsheet that I need to calculate with a formula and I am stuck to doing it manually right now.  In column A I have tonnage weight that needs to either subtract or add the difference of the two numbers in column D & E. And then divide that sum (D&E) by 2 before adding it back to column A.  If column D is greater than E, then add the difference and if E is greater than D, subtract the difference. I hope that makes sense. TIA