Difference between 2 collum

To give a small example.

I have 2 collums. In collum A there is always a value, an amount.

In collum B a value is added as the input grows, this value is always bigger then collum A.

Now I would like a cell to give me the difference between collum B and A, so B - A.

BUT I want this result only to include the difference if both collum are filled in.

Else the result will always be negative.

So only when there is a value in collum B, the value of cell in collum B - cell collum A and this for the entire collum.

I hope it makes any sense.