limit the different row in first page and the rest pages

[emilythird]

Is there anyone to help me with this ? i would like to show the very first 20 lines in the first page and in the other pages only 30 lines.

In the first page , i added the header band and the other pages don`t need the header band .

The problem is in the detail band , i made the setting in header expression like 

IF($V{PAGE_NUMBER==1},($V{GROUP_COUNT}/20==0),($V{GROUP_COUNT}/30==0)) 

It still shows the 20 lines　in the detail band .

   Thanks ahead,