calculate difference between current month and previous in Matrix report?

Hi,

i have one matrix 

like

P_Name     JAN   FEB     MARCH     APRIL   MAY JUNE JULY AUGUST....... DEC     (CURRENTMONTH vs PREVIOUS MONTH)

aaaa           10    20      15             20     0       0   0  0   0                    0               5 (april-march)

bbbbb         2     4        5                6      0        0   0  0  0                      0              1 (april-march)

kindly help me how to get last column. month column dynamically changes (depend upon month).