Generate Random Number2

namespace RandomNoGenerate2
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        int min,max,val=0;
        Random rdm;
        bool b;
        private void btn_RGenerate_Click(object sender, EventArgs e)
        {
            rdm = new Random();
            min = Convert.ToInt32(txt_min.Text);
            max = Convert.ToInt32(txt_max.Text);
            val = rdm.Next(min, max);
            textBox1.Text = val.ToString();
        }

        private void btn_Add_Click(object sender, EventArgs e)
        {
            b = false;
            rdm = new Random();
            min = Convert.ToInt32(txt_min.Text);
            max = Convert.ToInt32(txt_max.Text);
            val = rdm.Next(min, max);
            textBox1.Text = val.ToString();

            for (int i = 0; i < listBox1.Items.Count; i++)
            {
                if (val == Convert.ToInt32(listBox1.Items[i]))
                    b=true;
            }
            if(b)
                    textBox1.BackColor = Color.Red;
                else
                    textBox1.BackColor = Color.White;                   
             
        }
    }
}