Begginer, need help with a program that convers roman-> arabic numbers

Hello people, i have a problem making this program as it always prints out 0 at the end. If anyone can check the code i will be very grateful. THanks in advance.




Dim rimski, c, d As String
Dim broj, i, a, b As Integer


Private Sub Form_Activate()
a = 0
b = 0
broj = 0
rimski = Val(InputBox("vnesi rimski broj"))
rimski = UCase(rimski)
For i = 1 To Len(rimski)

Select Case Mid$(rimski, i, 1)
Case "I"
a = 1
Case "V"
a = 5
Case "X"
a = 10
Case "L"
a = 50
Case "C"
a = 100
Case "D"
a = 500
Case "M"
a = 1000
End Select

If i + 1 < Len(rimski) Then

Select Case Mid$(rimski, i + 1, 1)
Case "I"
b = 1
Case "V"
b = 5
Case "X"
b = 10
Case "L"
b = 50
Case "C"
b = 100
Case "D"
b = 500
Case "M"
b = 1000
End Select
Else
b = 0
End If

If b > a Then
broj = broj - a
Else
broj = broj + a
End If
Next i
Print broj



End Sub