Can not sum GridView column with a variable

Question

I want to sum GridView column value with a variable and show the result in second column.  I am trying to do this.But when I convert the GirdView column into an integer the following

 

"Input string was not in a correct format."  Error is occuring. 

 

Please help.

 

Here is my C# code:

 

for (int i = 0; i < GridView1.Rows.Count; i++)

{

long sum = openingBalace + Convert.ToInt64((GridView1.Rows[i].Cells[6].Text));

GridView1.Rows[i].Cells[8].Text = sum.ToString();

}

 

Thanks