Fill Row Number in DataGridview First Column in C#

public void FillRowNumber(DataGridView grd, DataGridViewRowPostPaintEventArgs e, Form frm)
{
string strRowNumber = (e.RowIndex + 1).ToString();
SizeF size = e.Graphics.MeasureString(strRowNumber, frm.Font);
if (grd.RowHeadersWidth < (int)(size.Width + 20)) grd.RowHeadersWidth = (int)(size.Width + 20);
Brush b = SystemBrushes.ControlText;
e.Graphics.DrawString(strRowNumber, grd.Font, b, e.RowBounds.Location.X + 15, e.RowBounds.Location.Y + ((e.RowBounds.Height - size.Height) / 2));
}