action based on cell selection

drdree73 

To achieve that, place the following codes on Sheet Module and to do that follow these steps...

1. Right click on Sheet Tab Name and choose View Code.

2. Paste the codes given below into the opened code window.

3. Save your file as Macro-Enabled Workbook.

Sheet Module Codes:

Private Sub Worksheet_SelectionChange(ByVal Target As Range)
If Target.CountLarge > 1 Then Exit Sub
Dim NumToSum    As Long
If Target.Column = 1 And Target.Row > 1 Then
    Range("C1").ClearContents
    If Target <> "" And IsNumeric(Target.Value) Then
        NumToSum = getNumber(Range("B1").Value)
        If NumToSum <> 0 Then
            Range("C1").Value = NumToSum + Target.Value
        End If
    End If
End If
End Sub

Function getNumber(ByVal str As String) As Long
Dim Matches As Object

With CreateObject("VBScript.RegExp")
    .Global = False
    .Pattern = "\d+"
    If .test(str) Then
        Set Matches = .Execute(str)
        getNumber = Matches(0)
    End If
End With
End Function

Please find the attached with codes in place.