Excel help

Hello,
Please help me with this
I want to limit my data entery in one cell.
for example the data enterd in cell A2 should be less than or equal to the value calcualted by formula
Suppose my formula is 2*100(50-48)
So the value in Cell A2 should not be more than the value calcualted from above formula. If value entered more then the value calucalted by the formula then it should give a error message.
any help in this regard.