Попытка деления на ноль - C#

public string[] stroki = { };
        public char bykva, cyfra;
        public int i = 0;
        public char per;
        public double otn;
        public string a;
        public Form1()
        {
            InitializeComponent();
        }
 
        private void button1_Click(object sender, EventArgs e)
        {
            string B = textBox1.Text;
 
            string[] new_md = new string[stroki.Length + 1];
            stroki.CopyTo(new_md, 0);
            stroki = new_md;
            stroki[stroki.Length - 1] = B;
            listBox1.Items.Add(B.ToString() + ", ");
            textBox1.Text = "";
        }
 
        private void button2_Click(object sender, EventArgs e)
        {
            double razn = 0;
            double minrazn = double.MaxValue;
            string a= "0";
            for (int i = 0; i < stroki.Length; i++)
            {
                for (int j = 0; j < stroki[i].Length + 1; j++)
                { 
                    if (char.IsLetter(stroki[i][j]))
                    { bykva++; }
 
                    if (char.IsNumber(stroki[i][j]))
                    {
                        cyfra++;
                    }
                    otn = bykva / cyfra;
 
                    razn = Math.Abs(1 - otn);
                    if (razn < minrazn)
                    {
                        minrazn = razn;
                        a = stroki[i];
                    }
                    MessageBox.Show(a.ToString());
                }
 
            }
        }
 
    }
}