How can I turn a minus result into a zero.

For example....

Column A = Stock Levels Needed /50
Column B = Actual Stock Levels /60
Column C = Plan to produce (A-B) /-10
Column D = Predicted Sales
Column E = Actual Sales /30
Column F = Predicted Stock (B+C-E) /20

If coulmn C returns a minus value then Column F returns an incorrect value.
The numbers after the / reflect the numbers my formulae returns. Where in
actual fact Column F should read 30 as there is 60 on stock, I only need to
keep 50 so I don't need to make any. I've sold 30 so I should have 30 left.
But because Column C is a minus value it takes that off too. If the result
of Column C is a positive value then it works fine.

Hope I've explained this good enough.
Thanks in advance.