I want calculation of next cell when i click in first cell

Question

i want calculation of next cell when enter on  first cell in datagridview

 

 

private void gvExpensesTaxses_CellEnter(object sender, DataGridViewCellEventArgs e)

{

decimal per;

decimal tamt;

foreach (DataGridViewRow row in gvExpensesTaxses.Rows)

{

per=Convert.ToDecimal(row.Cells["%(Per)"].Value);

tamt=Convert.ToDecimal(txtTotalAmt.Text);

decimal t =Convert.ToDecimal(tamt) / 100 * Convert.ToDecimal(per);

row.Cells["Amt"].Value = Convert.ToDecimal(t).ToString();

}

}

 

I get erroe like'Input string was not in a correct format.' 

in this line Plz help me ........................................... 

per=Convert.ToDecimal(row.Cells["%(Per)"].Value);