Output of the Program | Pointer to a Constant or Constant Pointer?

Predict the output of the below program.
 

6

Explanation: 
See following declarations to know the difference between constant pointer and a pointer to a constant. 
int * const ptr ---> ptr is constant pointer. You can change the value at the location pointed by pointer p, but you can not change p to point to other location.
int const * ptr ---> ptr is a pointer to a constant. You can change ptr to point other variable. But you cannot change the value pointed by ptr.
Therefore above program works well because we have a constant pointer and we are not changing ptr to point to any other location. We are only incrementing value pointed by ptr.
Try below program, you will get compiler error.