No overload for method 'add' takes 2 arguments error in C#

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ListBox

I try to get value int i and j in listbox, but then this error come out
No overload for method 'Add' takes 2 arguments. Can somebody help me please. I've been stuck at this error almost 2 days..

What I have tried:

private void Form10_Load(object sender, System.EventArgs e)
        {
            int i;
            int j;

            for (i = 1; i <= Form9.Npmax; i++)
            {
                for (j = Form9.Nsmin_mp; j <= Form9.Nsmax_oc; j++)
                {

                    listBox1.Items.Add(i.ToString(),j.ToString());

                }
            }
        }