Private Sub Worksheet_Change(ByVal Target As Range)
Dim c As Range, d As Range
Set c = Target: Set d = c.Offset(, 1)
If Intersect(Range("C:C"), c) Is Nothing Then Exit Sub
If Len(c.Formula) = 0 Then Exit Sub
If Not IsNumeric(c.Value) Then Exit Sub
If Not IsNumeric(d.Value) Then Exit Sub
On Error GoTo errh
Application.EnableEvents = False
d.Value = d.Value + c.Value
errh:
Application.EnableEvents = True
End Sub