What is the problem in my code?

C#, desktop application

I have a datagridview like this:

ID	Date	SoinikNo	APenName	Pension	Utsob
9	01/01/2017	L	A	3	2
16	05/02/2017	M	B	4	3
29	12/03/2017	L	A	5	7
33	17/04/2017	M	B	6	8
47	25/05/2017	L	A	3	5
52	13/06/2017	S	C	2	9
82	02/07/2017	S	C	1	12

I need a new datagridview like this:

Sl. No.	SoinikNo	APenName	Pension	Utsob
1	L	A	11	14
2	M	B	10	11
3	S	C	3	21

private void button1_Click(object sender, EventArgs e)

{

dataGridView2.Visible = true;

List _product = new List();

int Count = 1;

foreach (DataGridViewRow row in dataGridView1APen.Rows)

{

if (row.Cells[0].Value != "" && row.Cells[0].Value != null)

{

if (_product.Where(x => x.SoinikNo.ToLower() == row.Cells[0].Value.ToString().ToLower()).Any())

{

string Utsob = (Convert.ToInt32(_product.Where(x => x.SoinikNo.ToLower() == row.Cells[0].Value.ToString().ToLower()).Select(x => x.Utsob).FirstOrDefault()) + Convert.ToInt32(row.Cells[13].Value.ToString())).ToString();

string Pension = (Convert.ToInt32(_product.Where(x => x.SoinikNo.ToLower() == row.Cells[0].Value.ToString().ToLower()).Select(x => x.Pension).FirstOrDefault()) + Convert.ToInt32(row.Cells[12].Value.ToString())).ToString();

_product.Where(x => x.SoinikNo.ToLower() == row.Cells[0].Value.ToString().ToLower()).ToList().ForEach(x => { x.Utsob = Utsob; x.Pension = Pension; });

}

else

{

_product.Add(new ProductList

{

Sno = Count,

SoinikNo = row.Cells[2].Value.ToString(),

APenName = row.Cells[3].Value.ToString(),

Pension = row.Cells[12].Value.ToString(),

Utsob = row.Cells[13].Value.ToString()

});

Count++;

}

dataGridView2.DataSource = _product;

}