How do I subtract a value from a specific cell from a drop down box with list of numbers?

Hello Everyone,

How do I subtract a value from a specific cell from a drop-down box with a list of numbers? As shown on the image attached, I would like to choose a number from QTY dropdown box E2 (1 - 5) and then subtract the number from a specific cell e.g. C2 which currently has a value of 10.

image.JPG

Any help is greatly appreciated.
Many thanks in advance,