Creating calculator in C# windows application form but its showing 2 errors.

namespace WindowsFormsApplication10
{
    public partial class Form1 : Form
    {
       
        public Form1()
        {
            InitializeComponent();
        }
        
        public int a, b, c, d;
        public int e, f, g, h;
        public int option;
        private void button1_Click_1(object sender, EventArgs e)
        {
            textBox1.Text = textBox1.Text + button1.Text;
        }

        private void button2_Click(object sender, EventArgs e)
        {
            textBox1.Text = textBox1.Text + button2.Text;
        }

        private void button3_Click(object sender, EventArgs e)
        {
            textBox1.Text = textBox1.Text + button3.Text;
        }

        private void button4_Click(object sender, EventArgs e)
        {
            textBox1.Text = textBox1.Text + button4.Text;
        }

        private void button8_Click(object sender, EventArgs e)
        {
            textBox1.Clear();
        }

        private void add_Click(object sender, EventArgs e)
        {
            option=1;
            a = int.Parse(textBox1.Text);
            textBox1.Clear();
        }

        private void sub_Click(object sender, EventArgs e)
        {
            f = 11;
            option=2;
            b = int.Parse(textBox1.Text);
            textBox1.Clear();
        }

        private void equal_Click(object sender, EventArgs e)
        {
             if (g == 10 && textBox1.Text != "")
            {
                c = int.Parse(textBox1.Text);
                d = c + a;
                textBox1.Text = d.ToString();
            }
            else if (f == 11 && textBox1.Text != "")
            {
                  if (textBox1.Text != "")
                {
                    e = int.Parse(textBox1.Text);
                    h = e - b;
                    textBox1.Text = h.ToString();
                }
            }
           
            }
        }
    }