List box addition

I have create a listbox which contains a list of numbers representing times such as
1.21
2.34
4.12

i have a label at the bottom and i would like to perform a calculation on the numbers in the list box to add them together and display the total result in the label

like this maybe ...

VB Code:

Dim dbl As Double = Nothing
        Dim stritem As String
        For Each stritem In ListBox1.Items
            dbl += Convert.ToDouble(stritem)
        Next
        MessageBox.Show(dbl.ToString())