if minimum requirement isn't met

Hi I'm looking I have a problem and I'm looking for a formula.
I have an order sheet in excel and if that order on the sheet doesn't accumulate to a certain value an extra amount will be added to bring the order to the mimimum value.

e.g. if the minimum value is 100 and the order is only for 60, 40 will be added.
or if the minimum value is 100 and the order is for 70, 30 will be added.
The amount added could vary all the time.

Is there any formula anyone could think of to help me with this?

Thanks,
Ciaran