Formula

Hi, 

I need help making a formula. Have a document where I write in all the incoming and outgoing invoices for the company. The incoming (In) has one column with a the total sum (Summa) at the end and likevise for outgoing invoices (Ut). Next to them is a column that I tick in when it is payed. 
What I want is that when I tick in column F that the total sum will increase with the payment automatically. Do anyone know how to do that if it is posssible?