Setting iDisplayLength only for the first page

I currently have the following code that prepends a row to the start of the table, but only on the first page (I realise this is probably bad practice but I struggled to get fnAddTr etc. to work with server-side code). The problem is that iDisplayLength stays at 24 on page 2, and only on page 3 does it go up to 25. If you navigate back from page 3 to page 2, however, it's up to 25. How can I stop page 2 from defaulting to length 24?

"fnDrawCallback": function(oSettings) {
	if(oSettings._iDisplayStart == 0){
		$('#results tbody').prepend('<tr class="even"></tr>');
		$('#results tr.odd:last-child').remove();
		oSettings._iDisplayLength = 24;
	}else{
		oSettings._iDisplayLength = 25;
	}
}