Sum of Datagridview.

1. private void dataGridView1_RowPostPaint(object sender, DataGridViewRowPostPaintEventArgs e)
2. {
3. this.dataGridView1.Rows[e.RowIndex].Cells["Column1"].Value = (e.RowIndex + 1).ToString();
4. //--------------------for SUM-------------------
5. int sum = 0;
6. for(int i = 0; i < dataGridView1.Rows.Count-1; ++i)
7. {
8. sum =sum+int.Parse(dataGridView1.Rows[i].Cells[12].Value.ToString());
9. }
10. textBox23.Text = sum.ToString();
11. }

when i put cell[13] which is quantity and simple degit (For example 5 etc) it works properly and when i put cell[12] which is value (for Example 12.52 etc) i am geting error.
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An exception of type 'System.FormatException' occurred in mscorlib.dll but was not handled in user code

please help me....
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