WPF Listview c#

Hi,

I have a problem with a Listview. I get data from a database and I can select more lines, in the same time when I select the line I check a checkbox in a column.

Now the problem: I want to sum the numbers in a particolar column of my listview where the row is selected (where the rows is selected i have the check in the checkbox).

How can I take numbers inside the listview?

I want to press a button and to show the summary:

private void buttonSomma_Click(object sender, RoutedEventArgs e)
{
//I need help here, I don't know how to move inside listview
}

thank you for your kind attention.

Tullio

(Framework 3.5)