Make some first and last two lines of text should be bold in richtextbox

private void button1_Click(object sender, EventArgs e)
       {
           MakeBold(richTextBox1.Lines[0].Split(' ')[0]);
           MakeBold(richTextBox1.Lines[1].Split(' ')[0]);
           MakeBold(richTextBox1.Lines[3].Split(' ')[0]);
           MakeBold(richTextBox1.Lines[4].Split(' ')[0]);
       }

       private void MakeBold(string text)
       {
           int index = richTextBox1.Find(text);
           richTextBox1.Select(index, text.Length);
           richTextBox1.SelectionFont = new Font(richTextBox1.Font, FontStyle.Bold);
       }