EXCEL- Either or formula HELP!

Hi can someoine please help my dilemma is below;

I am trying to get an invoice total which is either and addition of figures from 3 different cells OR a single figure from one cell. Which figure the 'total' cell will use will depend on which cell is not blank.

EXAMPLE

Cell N1 =

(Cell K1+L1+M1) if there are figures in these cells (however not all 3 cells might have figures but we would still want to use their added total in Cell N1

OR
If all the above cells were blank then it would take a singular figure from Cell J1

I hope i have explained well enough.

Thanks

Alli