Export from datagridview to *.txt file only first 20 cell's letters

I'm using this code to export from datagridview to *.txt file

TextWriter sw =newStreamWriter(@"C:\fiscal.txt");int rowcount = dataGridView1.Rows.Count;for(int i =0; i < rowcount -1; i++){
sw.Write("{0,-20}", dataGridView1.Rows[i].Cells[0].Value.ToString());}
sw.Close();

But if my datagridview's cell is bigger than 20 letters i want to delete rest of them. And to export only my first 20 letters.