I have a pandas dataframe which are indexed by time, 

For example: 

```
Time                       Value
2010-01-01                 nan
2010-01-02                 nan
2010-01-03                 3
2010-01-04                 4
2010-01-05                 5
2010-01-06                 3
2010-01-07                 nan
2010-01-08                 nan
2010-01-09                 3
2010-01-10                 3
2010-01-11                 4
2010-01-12                 5
2010-01-13                 3
2010-01-14                 nan
2010-01-15                 nan

```

In this example, I would like to drop the first two and last two rows. But not the rows with nan in the middle. Is there a way to do this?