How can I reshape the following dataframe with pandas functions?

Question

How to reshape the following dataframe:

col1 | range| OK | NG  
-----------------------
A1   | B1   | 1 | 11     
A1   | B2   | 20| 12
A1   | B3   | 30| 13

into the following one?

col1 | range| class| count
-----------------------
A1   | B1   | OK  | 1     
A1   | B1   | NG  | 11  
A1   | B2   | OK  | 20
A1   | B2   | NG  | 12
A1   | B3   | OK  | 30
A1   | B3   | NG  | 13