Sort but neglect dashes / null

I want to sort the rows an a value. This value is a numeric value. But it can also be not available, so at that point I will echo a dash (-).

When I now sort this column, all rows with the dash are on top. And than the rows with value 1, 3, 6, 8, 10 are shown. How do I change this so that the dash (-) are always on bottom of the table?

At the moment I put in a maximum number, what puts them on the bottom. However I don't want this value to be shown to the user. So I need a hidden sort column, or a other sorting method.

Thanks in advance!

Visualizen:
This is standard:
-
-
1
2
3
4

I want:
1
2
3
4
-
-