forgot to pay interest on August GSTR 3B

I have filed my gstr 3b on 29-09-2017 and my tax liability was 22000 and late fee 1800 and I have paid 22000+ 1800 but I forgot to pay 18% interest 
what to do now ?
people are saying
1) you have to pay only 9 days interest while filing gstr 3 ( but date is not declared yet then ?)
2) no need to pay interest for July and August only penalty should be paid
3) you have to pay interest till the filing date of gstr 3 i.e. From 20 Sept to date of GSTR 3B of August ( date is not yet declared )

Which is correct. Please give me the sulution on this issue.