mathematical problem....help please

My landlord has just hit me with a back rent bill informing me that my rent should have been paid each four weeks and not cal/monthly as I have been doing.On checking my lease dated 29/10/2001 I find that he is correct, so no big deal.
BUT,I am having trouble confirming that his assesment is correct.The figures are;
a) rent is $270 per week.
b) I have been paying $1080 per cal/month.
c) 29/10/2001 'til 9/2/2004

Question,how much do I owe to pay my rent to 9/2/04.
No prizes,just my undying love if your figure is less than mine.TIA.