VBA code depending on criteria

I want macro for: I have 5 columns where I want like

COL 1 COL 2 COL 3 COL 4 COL5
JAN 13 4 1
JAN 12 8 7
FEB 13 7 4
FEB 14 7 5
MAR 44 10 7
now i want to calculate on col 5 = (sum of col2 values ) / (sum of col 3 values- sum of values col 4) which belongs to JAN and i want this solution to print on 1 cell of COL5

similarly for other months output should print on first cell belongs to FEB cell in col 1

Please i am very poor in this macro coding part . can someone help me to decode this one.

For better understanding of my problem i attaching image also