Random Data Set Needing Weird Values

Attached you will find a sample set of data from a spreadsheet that contains 26,000 values that I don't want to calculate by hand. I have include all the possible scenarios between week 1 and week2. I have also included what the answer should be in column C. The below information is what I need a formula written for to determine the answer. Any help would be greatly appreciated.

If both numbers are positive, then the answer is 0
If both numbers are negative, then the answer is 0
If both numbers are zero, then the answer is 0
If one positive and one negative, then sum two numbers
If positive sum take negative column value
If negative sum take positive column value