Yes..it works! Thank you!

But there is a problem in the code

Sub add()
Range("C8").Value = Range("B3").Value + Range("E3").Value
End Sub

In order to get the result, I must run the code every time manually.

What I want is, Real time update, means...
as soon as I enter a value in the cell B3 or E3, the value should be updated in the cell C8 automatically and immediately, without running the code manually.