Pandas: Concatenate from two rows in rolling window

I have a Series like this using Pandas

>>> kk
0        0
1     2976
2     5664
3     8640
4    11520

And I want roll a two-item window to get a new series like this:

>>> pp
0           NaN
1        0-2976 
2     2976-5664
3     5664-8640
4    8640-11520

As you can see I need to access multiple rows per group in the window What would be the function to get pp from ss where:

ss = kk.rolling(2)