how to clear the label Text while validating textbox("required field")

I ve 2 text box , a label and a button and 2 required field validators. (use to calculate sum)

TextBox: 

1) txtHeight (TextBox1)

2) txtWeight (TextBox2)

Button:

1) btnCalculateSum

Label:

1)lblShowSum

both textbox ve required field validator

Case: 

1)now when i inset the 2 in 1st textbox and 3 in 2nd textbox and press the calculate button it show the 6 in label. (work fine)

2) now i clear the weight textbox and press calculate button, now the problem comes ...

Red * showing dat weight is required but lblShowSum is still displaying the previous sum.

how to clear that label, if the user is not entering values in both textbox

 

thanks in advance.... Go to the complete details ...