how to Add column cell value to previos cell value and get result

[fager elsaba7]

this my code
iwant to add column cellvalue in datagridview to previouse number of cells user insert
the summ variable always equal zero


any help

 Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        year = TextBox1.Text
        month = TextBox2.Text
        noofmonths = TextBox3.Text
        Dim summ As String
        summ = 0
        Dim rowindex As Integer



        For Each row As DataGridViewRow In DataGridView1.Rows
            If row.Cells.Item(1).Value = TextBox1.Text Then
                If row.Cells.Item(2).Value = TextBox2.Text Then
                    rowindex = row.Index




                    For i As Integer = rowindex To noofmonths And i <> 0

                        summ = row.Cells.Item(3).Value + summ


                        rowindex = row.Index - 1

                        i = i - 1




                    Next
                    summ = summ / noofmonths.ToString

                    rowindex = rowindex + noofmonths
                    Dim actie As String = row.Cells(3).Value.ToString()













                    row.Cells(4).Value = summ

                    MsgBox(summ)

                Else

                    MsgBox("Item not found")

                End If
            End If


        Next