Adding ASP.Net Labels which contains value and Empty String using C#

I have a label that adds the values of other labels and shows the total. But the error I am having is that if for example a label contains and empty value, I will have an error like input was not in the correct format.

How do I make it in such a way that even if a Label control contains a null or an empty value, it will still add up the values of other labels and show the total.

For example, label1 + label2 + label3 + label4 + label5 + label 6

Then if Label1 = 5

Label2 = 4

Label3 = "" (or empty and does not have any value)

Label4 = 10

Label5 = 1

Labe6 = "" (or empty and does not have any value)

Then LabelTotal = 20 (i.e., 5 + 4 + 10 + 1)

I have this code on my PageLoad event

lblTotal.Text = (int.Parse(Label1.Text) + int.Parse(Label2.Text) + int.Parse(Label3.Text) + int.Parse(Label4.Text) + int.Parse(Label5.Text) + int.Parse(Label6.Text)).ToString("###,##");