All wrong! As i mentioned in the comment to the question, you have to use proper data type!
Seems, you want to add a sum of two rows in the third one - for each column. So, your code can be simplified to:
for(int i=0; i<12; i++)
dataGridView1.Rows.Cells[i].Value = dataGridView1.Rows.Cells[i].Value + dataGridView1.Rows.Cells[i].Value;