datagridview sum cels

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C#

DataGridView

Hi

I have one problem with datagridview. I want to multiplying two cells and display the result in third cell

example:

A     | B  |  C
10.50 | 2  | 21,00
5.20  | 4  | 20,80
7,3   | 5  | 36,50

after that i wanna to sum C column and display that in textBox.
Column A is type decimal
Column B is integer
Column C should be decimal too

how to do that and how to display decimal value in textbox?

This is what I have trie

foreach (DataGridViewRow item in dataGridView1.Rows)
{
  int n = item.Index;
  dataGridView1.Rows[n].Cells[2].Value = 
      (Double.Parse(dataGridView1.Rows[n].Cells[0].Value.ToString()) +
      Double.Parse(dataGridView1.Rows[n].Cells[1].Value.ToString())).ToString();
}

textBox1.Text = "0";
foreach (DataGridViewRow item in dataGridView1.Rows)
{
  int n = item.Index;
  textBox1.Text = (Double.Parse(textBox1.Text.ToString()) +     
          Double.Parse(dataGridView1.Rows[n].Cells[2].Value.ToString())).ToString();
}

this code should sum cells, but i want to multiplying cells. i was trying to do anything!