How to concat one word and one number which entered in two different textbox?

     private void button1_Click(object sender, EventArgs e)
        {
            System.Text.StringBuilder sb = new System.Text.StringBuilder();
            string firststring;
            string secondstring;

            firststring = txtfirst.Text;
            secondstring = txtsecond.Text;
           if (firststring.Length > secondstring.Length)
            {
                int count = 1;
                foreach (char ch in firststring)
                {
                    int i = 1;
                    foreach (char chh in secondstring)
                    {
                        if (i == count)
                        {
                            sb.Append(ch).Append(chh);
                         
                        }
                        i++;
                    }
                    if (count > secondstring.Length)
                    {
                        sb.Append(ch);
                    }
                    count++;
                }

                MessageBox.Show(sb.ToString());
            }
            else
            {
                int count = 1;
                foreach (char ch in secondstring)
                {
                    int i = 1;
                    foreach (char chh in firststring)
                    {
                        if (i == count)
                        {
                            sb.Append(chh).Append(ch);
                        
                        }
                        i++;
                    }
                    if (count > firststring.Length)
                    {
                        sb.Append(ch);
                    }
                    count++;
                }

                MessageBox.Show(sb.ToString());
            }
        }

firststring : ABCD
secondstring : 9090969698

O/P :A9B0C9D0969698

Thanks.