Average depending of parameter given - reporting-services

Question

I have a month parameter called DimTiempoMes So I when I run preview of report I select that month in combo box it can be from 1 to 12.
If I select 1 it return value of Fields!InventarioDirecto_Monto.Value of month 1
If I select 3 it return value of Fields!InventarioDirecto_Monto.Value of month 1,2 and 3 etc...
Now I want to get Average depending of selected Fields!InventarioDirecto_Monto.Value
For Example: If I selectDimTiempoMes with value10, formula will be for example:
InventarioDirecto_Monto Value / DimTiempoMes.Value
Execution: Sum(1,2,3,4,5,6,7,8,9,10) / 10
So I try next expression:
=SUM(Fields!InventarioDirecto_Monto.Value) / CINT(replace(left(Split(Parameters!DimTiempoMes.Value,"[").GetValue(3),2),"]",""))
Problem is SUM(Fields!InventarioDirecto_Monto.Value)always come with sum of 12 months values, so it always sum values of 12 months
Question is, How can I get only values that Fields!InventarioDirecto_Monto.Value return depending of selected parameter DimTiempoMes?
Some Data:
In this image I have 3,719 in first column because I use DimTiempoMes as 1: so my colum Acum who have expression should be 3,719 too because SUM(Fields!InventarioDirecto_Monto.Value) equals to 3719 and divisor equals to 1 because I search 1. So 3,719/1 = 3791 and I getting 40,593
Other Example: if I use DimTiempoMes as 4
There I should get 3719 + 3639 + 3378 + 2999 / 4 = 3433.73 and I receive 10148
Update:
I also try adding custom code like:
Function Promedio(ByVal items As Object(),ByVal meses As Object(), ByVal Valor1 As integer) As Decimal
If items Is Nothing Then
Return Nothing
End If
Dim suma As Decimal = New Decimal()
Dim ct as Integer = New Integer()
suma = 0
For Each mes As Object In meses
IF CINT(mes) <= Valor1
Then ct = 1
For Each item As Object In items
If (ct = CINT(mes))
Then suma += Convert.ToDecimal(item)
End If
ct += 1
Next
End If
Next
suma = suma/valor1
If (Valor1 = 0) Then return 0 else return suma
End Function
And Expression something like:
=Code.Promedio(LookupSet(Fields!Dim_ConceptosInventario_.Value, Fields!Dim_ConceptosInventario_.Value, Fields!InventarioDirecto_Monto.Value, "DS_SubInventarioDirecto"),LookupSet(Fields!Dim_ConceptosInventario_.Value, Fields!Dim_ConceptosInventario_.Value, Fields!nPeriodo.Value, "DS_SubInventarioDirecto"),CINT(replace(left(Split(Parameters!DimTiempoMes.Value,"[").GetValue(3),2),"]","")) )
But I don´t get desire results