dataGridView 多行选择取列数值相加

string[] str = new string[dataGridView1.Rows.Count];
int Sum = 0;
for(int i=0;i<dataGridView1.Rows.Count;i++)
{
if(dataGridView1.Rows[i].Selected==true)
{
str[i] = dataGridView1.Rows[i].Cells[2].Value.ToString();
Sum = Convert.ToInt32(str[i].ToString())+Sum;
}
}