Try:
Sub IndexToStringConvertiA1C1()
Dim str As String
Dim Riga As Long
Dim colonna As Long
Riga = Application.InputBox(prompt:="Riga", Type:=1)
colonna = Application.InputBox(prompt:="Colonna", Type:=1)
Convert = IndexToString(Riga, colonna, str)
MsgBox str
End Sub
Function IndexToString(row As Long, col As Long, strResult As String)
strResult = ""
If col > 26 Then
strResult = Chr(Asc("A") + Int(col - 1) / 26 - 1)
End If
strResult = strResult & Chr(Asc("A") + ((col - 1) Mod 26))
strResult = strResult & row
End Function