如何用R计算相同月份和不同年份之间的差异

I have this data frame called, df. Rows are Months and rowname is Year. I would like to order the months from Jan-December and would like to calculate the difference between the same months in different years in %. For example, I would like to know the % difference between 2009 and 2008 for the Jan, Feb, etc. Do this for all the months.I have this data frame called, df. Rows are Mon