Ошибка в калькуляторе - C#

        public int iNum;
 
        public Form1()
        {
            InitializeComponent();
        }
 
        private void button1_Click(object sender, EventArgs e)
        {
            textBox1.Text += "1";
        }
 
        private void button2_Click(object sender, EventArgs e)
        {
            textBox1.Text += "2";
        }
 
        private void button4_Click(object sender, EventArgs e)
        {
            iNum = Convert.ToInt32(textBox1.Text);
            textBox1.Text = "";
        }
 
        private void button3_Click(object sender, EventArgs e)
        {
            textBox1.Text += iNum;
        }