Query regarding paying penalty for late filing of gstr 1

Hi All,

I am a service provider with an annual turnover of less than 1.5 cr. By mistake I opted for monthly filing of GSTR1.  

I filed the gstr1 for April and May late , on 27th June. April was nil return while May had one invoice with IGST . 

Just wanted to check how much penalty do I need to pay and do I need to pay it before I file my GSTR 3B for June ?? 

Please please help... Really confused.

Thanks and regards

Anita