IF Statement PROBLEM

Hi,

I have created a budget sheet and would like to carry forward any money which is left over in one financial period to another. What condition can i use to carry forward the amount, as i do not want to carry forward IF the amount is negative.

I want to be able to put a formula in place so that i do not have to go back and calculate the amount myself.

I have attached a sample worksheet for you to observe.

Thank you for you help.