Datagridview add column data if it contain number - vb.net

Question

I've a datagridview like this:
------------------------
| S.N |Data1 | Data2|
| 1 | - | 10 |
| 2 | 4 | 2 |
| 3 | 2 | - |
| 4 | 9 | - |
I want result like this:
------------------------
| S.N |Data1 | Data2|
| 1 | - | 10 |
| 2 | 4 | 2 |
| 3 | 2 | - |
| 4 | 9 | - |
| total | 15 | 12 |
-----------------------
I've tried this:
Dim data1 As double = 0
Dim data2 As double = 0
For i As Integer = 0 To DataGridView1.RowCount - 1
data1 += Val(CDbl(DataGridView1.Rows(i).Cells(1).Value))
data2 += Val(CDbl(DataGridView1.Rows(i).Cells(2).Value))
Next
Dim rows As String() = {"Total", data1, data2}
DataGridView1.Rows.Add(rows)
But it has shown an error:
Can I extract number only from datagridview and display sum of them
and add to the last row?

Answer 1

And now I get my answer:
Dim data1 As double = 0
Dim data2 As double = 0
For j As Integer = 0 To DataGridView1.RowCount - 1
If Regex.IsMatch(DataGridView1.Rows(j).Cells(1).Value, "^[0-9 ]+$") Then
data1 += Val(CDbl(DataGridView1.Rows(j).Cells(1).Value))
End If
If Regex.IsMatch(DataGridView1.Rows(j).Cells(2).Value, "^[0-9 ]+$") Then
data2 += Val(CDbl(DataGridView1.Rows(j).Cells(2).Value))
End If
Next
Dim rows As String() = {"Total", data1, data2}
DataGridView1.Rows.Add(rows)

Answer 2

This checks for nulls and ensures the data is numeric.
Dim data1 As double = 0
Dim data2 As double = 0
For i As Integer = 0 To DataGridView1.RowCount - 1
If Not IsDbNull(DataGridView1.Rows(i).Cells(1).Value) AndAlso IsNumeric(DataGridView1.Rows(i).Cells(1).Value) Then data1 += Val(CDbl(DataGridView1.Rows(i).Cells(1).Value))
If Not IsDbNull(DataGridView1.Rows(i).Cells(2).Value) AndAlso IsNumeric(DataGridView1.Rows(i).Cells(1).Value) Then data2 += Val(CDbl(DataGridView1.Rows(i).Cells(2).Value))
Next
Dim rows As String() = {"Total", data1, data2}
DataGridView1.Rows.Add(rows)