HTML- Populating dropdown form select according to number of checked Checkboxes

How to populate form select option according to checked checkboxes. I have 5 checkboxes in my HTML page. If i check 3 checkboxes then dropdown select should show options as 1,2,3 similarly if i have 5 checkboxes selected then form select options should show a list of values 1,2,3,4,5.