Math problems/ operands from listbox

I need a code that would take numbers from a list box and solve them when i press add or multiply I have been working on this for hours and can't get it right......please help.
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What code do you have thus far?

As I understand It's should be like that
        private void buttonPlus_Click(object sender, EventArgs e)
            Int32 sum = 0;
            foreach (string s in listBox1.Items)
                Int32 value;
                if (Int32.TryParse(s, out value))
                    sum += value;

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