[R] Mean(s) from values in different row?

[SML]

Hello:

Is there a way to get a mean from values stored in different rows? 

The data looks like this:
  YEAR-1, JAN, FEB, ..., DEC
  YEAR-2, JAN, FEB, ..., DEC
  YEAR-3, JAN, FEB, ..., DEC

What I want is the mean(s) for just the consecutive winter months:
  YEAR-1.DEC, YEAR-2.JAN, YEAR-2.FEB
  YEAR-2.DEC, YEAR-3.JAN, YEAR-3.FEB
  etc.

Thanks.