Running totals

I have a spreadsheet that has days, targeted sales, actual sales

I want to have a function that calculates each day how much I am above or below target. So on day 6 I have 6 * Target - Sum Actual.

Example:

Day 1 2 3 4 5 6
Actual £92 £81 £142 £175 £60 £105
Target £100 £100 £100 £100 £100 £100
+/- £8 £27 £15 £90 £50 £55

I would like to know how to calculate the +/- row without having to do it manually. It should be a daily running total.

Any help would be greatly appreciated.

Thank you