Try this
Sub CNV()
Dim c As Integer
On Error GoTo ExitProc
c = InputBox("Enter Value")
ActiveCell = "=A1*" & c
Exit Sub
ExitProc:
fb = MsgBox("You have not inserted anything. You want to
continue?", vbYesNo)
If fb = vbYes Then CNV
If fb = vbNo Then Exit Sub

End Sub