totalling of subtotal values from the listed data

Hai to all, i want a formula which totals all subtotal values from the list of data

example
product month total
A April 250
B April 500
total 750
A May 200
B May 300
Total 500

ineed subtotal as =750+500=1250 (sum of subtotals)

this i need any solution

in the last cell of same column i need a formula showing link of subtotals

its just example, actually subtotals obtained by sum function. i need the total value of each month to be added in last. and when i click on that cell it should show me from which totals grand total arrived