I have a data frame 'df' which has a multilevel index ('STK\_ID','RPT\_Date'):

```
    sales        cogs    net_pft
STK_ID RPT_Date                                   
600809 20120331  2214010000   509940000  492532000
       20111231  4488150000  1077190000  780547000
       20110930  3563660000   850789000  707537000
       20110630  2894820000   703883000  658625000

```

Some code:

```
>>> df.index.names
['STK_ID', 'RPT_Date']

```

now I want to get the RPT\_Date column's series value (20120331,20111231,20110930,20110630) by: 

```
>>> df['RPT_Date'] # not work

```

How do I get that data?