C# Insert Item to ListBox with alternation method

I have for example five items in a ListBox and all five are selected. Now I would like to add a new one to each marked item with the text "IZ+20" and "IZ-20" in alternation. It should look like that:

1. X-200 Y+130 R0 FAUTO
2. IZ+20 R0 FMAX
3. X+200 Y+160 R0FMAX
4. IZ-20 R0 FMAX
5. X-200 Y+160 R0 FAUTO
6. IZ+20 R0 FMAX
7. X+200 Y+190 R0FMAX
8. IZ-20 R0 FMAX
9. X-200 Y+1-0 R0 FAUTO
10. IZ+20 R0 FMAX

How can I make this? Thank you...

My Code:

for (int n = listBox1.Items.Count - 1; n >= 0; --n)
{
    string select = "X";
    if (listBox1.Items[n].ToString().Contains(select))
    {
        listBox1.SetSelected(n, true);
        listBox1.Items.Insert(listBox1.SelectedIndex, "IZ+20");
        //and now "IZ-20 R0 FMAX"???
        listBox1.ClearSelected();
    }
}

You could also use a Queue for that job, enque both items in the beginning, then deque insert and enque again the same item, this way they will rotate and you will have the other alternating item at the next insertion attempt

using System.Collections.Generic;

Queue<string> insertionItems = new Queue<string>();
insertionItems.Enqueue("IZ+20 R0 FMAX");
insertionItems.Enqueue("IZ-20 R0 FMAX");


for (int n = listBox1.Items.Count - 1; n >= 0; --n)
{
    string select = "X";
    if (listBox1.Items[n].ToString().Contains(select))
    {
        listBox1.SetSelected(n, true);
        string item = insertionItems.Dequeue(); // removes first element
        listBox1.Items.Insert(listBox1.SelectedIndex, item);
        insertionItems.Enqueue(item); // appends first element at end
        listBox1.ClearSelected();
    }
}

at the second time the if-clause is entered the queue will yield the second element: "IZ-20 R0 FMAX" which will be appended again at the end. This will alternate the insertion elements