Drawing a line from the previous point c#

So I was messing around in c# making a project for school and I ran into this road block. How do I store an older line's variable/coordinate (CF) as the next lines new variable /coordinate (AF)

    public void button1_Click(object sender, EventArgs e)
    {
        if (n < 1)
        {
            Double AD = 0;
            Double BD = 0;
            n++;
        }
        else if (n > 1)
        {
            Double AD = 0;
            Double BD = 0;
            n++;
        }
        using (var g = Graphics.FromImage(pictureBox1.Image))
        {
            int L = Convert.ToInt32(numericUpDown1.Value);
            int T = Convert.ToInt32(numericUpDown2.Value);
            Double TH = System.Convert.ToDouble(T);
            Double LD = System.Convert.ToDouble(L);
            float AF = System.Convert.ToSingle(AD);
            float BF = System.Convert.ToSingle(BD);
            float CF = System.Convert.ToSingle(CD);
            float DF = System.Convert.ToSingle(DD);
            double THP = (TH / 180) * (Math.PI);
            DD = Math.Sin(THP) * LD + AF;
            CD = Math.Cos(THP) * LD + BF;
            g.DrawLine(Pens.Blue, AF, BF, CF, DF);
            pictureBox1.Refresh();
        }


    }