Cell validation guidance needed

[dckummer]

I need to put validation on a calculated cell which would prevent the calculated cell's value to equal a number less than zero. E.g. A1+B1=C1. C1 is calulated. A1 and B1 are where a user will input data. How can i prevent users from inputing data which would make cell C1<0.

Thanks

 

[RichardMGreen]

I think conditional formatting would probably be the best bet.
It will allow the user to enter values that would cause a less than zero result, but will flag it up.