Creating an addition and subtracting forumla in Numbers for Mac

how do I enter a sum forumla in numbers for mac. I'm creating a financial spreadsheet and need to ask the spreadsheet to calculate the new balance after subtracting and adding sums. For example: I want to calculate the running total in E column. I have an amount in E3 the spreadsheet will need to subtract the amount in c4 and add the amount in d4 to achieve the current amount. Everytime I've tried to do this I've received an error message