Adding 2 listbox and the sum display at the label

Question

I would like to ask how to add two listbox and the sum is in the label:

When i check the first listbox it sum but when i checked the other checkbox the label resets to 0 and summing up other numbers in the 2nd listbox i checked

My code on first listbox:
   protected void add(object sender, EventArgs e)
    {
        int sum = 0;
        int test = Int32.Parse(lbl_.Text);
        for (int j = 0; j <= pizzalist.Items.Count - 1; j++)
        {
            if (pizzalist.Items[j].Selected)
            {
                test += Convert.ToInt32(pizzalist.Items[j].Value);
            }
        }
        lbl_.Text = test.ToString();

My 2nd code in other listbox:
    protected void Button5_Click(object sender, EventArgs e)
    {
        int sum = 0;
        //   for (int j = 0; j <= waterlist.Items.Count - 1; j++)
        for (int j = 0; j <=  waterlist.Items.Count -1; j++)
        {
            if (waterlist.Items[j].Selected)
            {
                sum += Convert.ToInt32(waterlist.Items[j].Value);
            }
        }
        
        lbl_.Text = sum.ToString();
    }

Answer 1

Few things:

1. you need to read the label in second function and add the sum variable to make it work as expected - now you are changing the label without checking what value it has; more: you can try to make global variable with sum of items
2. use TryParse, or at least Parse with try-catch, Convert is a weak idea
3. try to use foreach instead of for
4. you can even use Linq to quickly sum selected items