C#实现dataGridView行的合计

private void button9_Click(object sender, EventArgs e)
        {
            string MJ,ym;
            float a = 0, b = 0, sumMJ = 0, sumYM = 0;
                       int i = 0;        //行索引
                       for (i = 0; i < dataGridView1.Rows.Count-1; i++)  //循环
                       {
                          
                               ym = dataGridView1.Rows[i].Cells["ychmi"].Value.ToString();
                               MJ = dataGridView1.Rows[i].Cells["mj"].Value.ToString();
                               a = float.Parse(ym);
                               b = float.Parse(MJ);
                               sumMJ = sumMJ + b;
                               sumYM = sumYM + a;
                       }
                       label16.Text = sumYM.ToString();
                       label19.Text = sumMJ.ToString();

        }