Sub DoWork()
    Dim n As Long
    n = 1&
    Do While Sheet1.Cells(1, n + 1&).Value <> ""
        n = n + 1&
    Loop

    Dim row As Long, col As Long
    For row = 1 To n
        For col = 1 To n
            Sheet2.Cells(row, col).Value = Sheet1.Cells(1, row).Value + Sheet1.Cells(2, col).Value
        Next col
    Next row
End Sub