Function SUMIF_Extended(sheet1name$, sheet2name$, sumcell$, condcell$, condition) As Long
    On Error Resume Next: Application.Volatile True
    Dim WB As Workbook, sh As Worksheet: Set WB = Application.Caller.Parent.Parent
    sh1_ind& = WB.Worksheets(sheet1name$).Index
    sh2_ind& = WB.Worksheets(sheet2name$).Index
    For i& = sh1_ind& To sh2_ind&
        Set sh = WB.Worksheets(i)
        If sh.Range(condcell$).Cells(1) = condition Then
            SUMIF_Extended = SUMIF_Extended + Val(sh.Range(sumcell$).Cells(1))
        End If
    Next
End Function