I am trying to make a spreadsheet for mileage expenses

for example
if D4 is the amount i was paid
and D3 is the amount of fuel i Paid for
I need to know how much is left for wear and tear costs D5

I can not get this sum to work, can you help with the formula?

eg: D4 =100
D3 = 50
D5 needs to automatically work out the difference so D5 = 50.