Option Explicit

Sub test()

    Dim LastRow As Long, i As Long

    'Set you sheet
    With ThisWorkbook.Worksheets("Sheet1")

        LastRow = .Cells(.Rows.Count, "A").End(xlUp).Row

        'Start from row 2
        For i = 2 To LastRow

            If Not IsEmpty(.Range("A" & i).Value) Then

                .Range("B" & i - 1).Value = .Range("B" & i).Value + .Range("B" & i - 1).Value

            Else

                .Range("B" & i + 1).Value = .Range("B" & i).Value

            End If

        Next i

    End With

End Sub